ADAPTIVE
T Helper (Th) Cells, T-Independent Antigens, T Regulatory (Treg) Cells, Thymocytes
Innate Immunity:
Includes barrier defenses (e.g., skin), phagocytic cells (macrophages, neutrophils), and preformed soluble factors (complement proteins).
Responses are general, with entire pathogen classes eliciting similar responses.
Responses are unchanging over time.
Adaptive Immunity:
Characterized by specificity for individual microbes and pathogens.
Exhibits memory of prior antigen exposures.
Responses enhance upon repeated exposure.
Represents a more tailored, pathogen-specific response compared to innate immunity.
Requires time (several days) after initial exposure to mount a response.
Can provide lifelong immunity against reinfection.
Divided into cell-mediated immunity (T cells) and humoral immunity (antibodies by B cells).
T Lymphocytes (T cells):
Responsible for cell-mediated adaptive immunity.
Divided into cytotoxic T cells and helper (Th) T cells.
Cytotoxic T cells: Kill target cells infected with intracellular pathogens (e.g., viruses, bacteria) and cancerous cells.
Helper T (Th) cells: Control the immune response by secreting cytokines (signaling molecules for communication between immune cells).
Subpopulations exist based on cytokine secretion profiles.
B Lymphocytes (B cells):
Produce antibodies (immunoglobulins).
Antibody roles include:
Labeling targets for phagocytosis.
Neutralizing viruses and toxins.
Blocking microbe adhesion to tissues.
Lymphocyte Receptor Gene Rearrangement:
A key difference between adaptive immunity lymphocytes and innate immunity cells.
Genes coding for lymphocyte receptors rearrange during T-cell or B-cell development.
Each lymphocyte is equipped to respond to a unique epitope associated with a specific pathogen.
Adaptive Immune System Diversity:
The adaptive immune system represents an immune army capable of combating any conceivable infectious threat due to slightly different receptor specificities on each lymphocyte.
Clonal Expansion:
Only lymphocytes responsive to epitopes on invading pathogens are activated and proliferate.
Proliferating lymphocytes give rise to populations of cells with genetically identical receptors specific for the same epitope.
Delay in Adaptive Response:
The characteristic delay between initial infection and an effective adaptive response arises because pathogen-responsive lymphocytes undergo massive clonal expansion to reach the necessary number of effector cells.
Effector Cells vs. Memory Cells:
Most progeny from clonal expansion are effector cells (cytokine-secreting/cytotoxic T cells or antibody-producing B cells).
A small proportion become memory cells, which enter a quiescent state and are long-lived.
Memory cells activate rapidly upon re-encountering their specific epitope, resulting in a swifter, greater magnitude response than the primary response.
Lymphocyte Production (Differentiation):
Begins early in fetal development.
Progenitors of T and B cells appear in the fetal liver as early as 8 weeks of pregnancy.
Bone marrow becomes the primary lymphocyte source later in fetal development and throughout adult life.
Lymphoid Progenitor Cells:
The earliest precursors of T and B lymphocytes are generated by asymmetric division of lymphoid progenitor cells.
Asymmetry refers to one daughter cell taking on lymphocyte characteristics while the other retains stem cell-like plasticity.
T-Cell Differentiation:
T-cell precursors leave the bone marrow and migrate to the thymus.
The thymus is divided into the outer cortex and inner medulla.
T-cell precursors become known as thymocytes once in the thymus.
Thymocytes enter at the cortico-medullary junction and migrate toward the thymic cortex.
During their 3-week migration, differentiation processes occur:
Rearrangement of T-cell receptor (TCR) genes.
Changes in expression of thymocyte cell surface markers.
Selection of thymocytes with functional receptors.
Deletion of thymocytes with self-reactive potential.
Interactions with stromal cells (macrophages, dendritic cells, fibroblasts, thymic epithelial cells) are critical for these processes.
Major stages of T-cell development: double-negative (DN), double-positive (DP), and mature T cells.
CD Markers
Key cluster of differentiation (CD) markers are found on the surface of immune cells, including lymphocytes.
CD markers provide a fingerprint, allowing classification of T and B cells and their developmental stage.
Bound by fluorescent-tagged antibody reagents; the number of lymphocytes positive for each marker can be determined using flow cytometry.
Routinely used in the diagnosis of leukemias, lymphomas, and immunodeficiency diseases such as AIDS.
Double-Negative (DN) Stage:
Cytotoxic T cells (Tc) and Th cells are identified by differential expression of CD4 and CD8 markers.
CD4 identifies Th cells, while CD8 identifies cytotoxic T cells.
Early thymocytes lack both CD4 and CD8, thus known as double-negative (DN) thymocytes.
DN thymocytes aggregate in the outer cortex, proliferating under the influence of cytokines like interleukin-7 (IL-7).
TCR Gene Rearrangement (DN Stage):
Rearrangement of genes coding for the TCR begins during the DN stage.
Random shuffling and altering of TCR genes yields expression of a unique antigen receptor by each thymocyte, creating a highly diverse T-cell population.
TCR Structure:
The TCR comprises two transmembrane proteins: alpha (α) and beta (β) chains.
Each chain possesses an intracellular signaling domain, a membrane-spanning domain, and an extracellular domain tipped with a variable region.
Variable regions of α and β chains are responsible for antigen recognition and undergo rearrangement.
TCR Gene Organization:
Before rearrangement, TCR β chain genes (chromosome 7) are divided into V, D, and J sections, each with multiple segments.
Enzymes in DN thymocytes clip genomic DNA and stitch it back together, randomly aligning one V, one D, one J segment, and constant region segment.
Imprecise cleavage and ligation enhance TCR diversity but may result in nonfunctional TCRs.
Strategies for Functional TCR β Chain:
T cells are diploid, so β chain genes on both copies of chromosome 7 simultaneously undergo rearrangement, doubling the probability of a functional β chain.
Enzymes mediating TCR gene rearrangement continue clipping and stitching variable-region DNA until a functional TCR is generated.
Pre-TCR Formation:
To test functionality of TCR β chains, a surrogate α chain (no variable region) is temporarily expressed, forming a pre-TCR with the rearranged β chain.
If the DN thymocyte expresses a functional β chain, the pre-TCR provides survival signals.
Gamma-Delta (γδ) T Cells:
Early on, some thymocytes rearrange and express gamma (γ) and delta (δ) TCR chains when there isn't a productive rearrangement of DNA coding for a β chain.
This unique population (10% or less) is γδ T cells.
γδ T cells proceed down an alternate developmental pathway, typically remaining negative for both CD4 and CD8.
Dominant T-cell population in the skin, intestinal epithelium, and pulmonary epithelium.
Tasks include wound healing and protection of the epithelium.
Do not require antigen presentation by major histocompatibility complex (MHC) proteins.
Represent an important bridge between innate and adaptive immunity.
Double-Positive (DP) Stage:
Successful pre-TCR signaling due to a functioning TCR β chain initiates rearrangement of the TCR α chain.
α chain gene variable regions are divided into V and J sections.
α chain rearrangement is mechanistically identical to β chain rearrangement.
The appearance of a functional α chain suppresses further TCR gene rearrangements.
Thymocytes become CD4-positive (CD4+) and CD8-positive (CD8+), known as double-positive (DP) thymocytes.
Allelic Exclusion:
Thymocytes are diploid cells with the potential to express two different α and β chains.
However, each T cell recognizes only one antigenic peptide.
Once a functional version of the β and α chains is produced, expression of the corresponding TCR gene on the opposite chromosome is permanently shut off.
CD3/TCR Complex:
The TCR α and β chains occur in a complex with six other molecules common to all T cells, known collectively as CD3/TCR complex.
The six chains of the nonspecific CD3 portion of the complex assist in intracellular signaling when an antigen binds to the TCR.
These chains occur in three pairs: delta-epsilon (-), gamma-epsilon (-), and a tau-tau (-) chain.
MHC Restriction:
T-cell receptors do not recognize peptide antigen alone. Peptide antigen must be held in an MHC molecule, or “presented” to T cells.
This property of T cells is established during the DP stage of thymocyte development.
Positive Selection:
Gene rearrangement produces TCRs with nearly unlimited specificities; each receptor must be tested for its ability to interact with MHC.
Thymocytes encounter stromal cells in the thymic cortex that express MHC class I and II proteins.
Interactions between the DP thymocyte’s TCR and stromal MHC determine the thymocyte’s fate:
TCRs that bind with very high affinity to MHC, or those that fail to bind to MHC at all, are induced to die by apoptosis.
Only those DP thymocytes with receptors that bind moderately to MHC survive.
T cells with receptors that bind to MHC too strongly have a high potential to react with self-antigens.
TCRs that don’t bind to MHC can’t function as mature T cells.
CD4 and CD8 Role in MHC Restriction:
Two important molecules for establishing the MHC restriction of T cells are CD4 and CD8, which bind to MHC class I and class II molecules, respectively.
During the DP stage, thymocytes express both CD4 and CD8.
Depending on which class of MHC molecule a positively selected thymocyte recognizes, the expression of the opposite marker begins to decrease substantially.
Thymocytes possessing TCRs that recognize MHC-II express CD4, whereas those that bind to MHC-I express CD8.
Stable expression of either CD4 or CD8 and loss of expression of the opposite marker signify the entrance of the thymocyte into the next phase of differentiation, the “single-positive” (SP) stage.
Single-Positive (SP) Stage:
Thymocytes that survive positive selection must overcome one final developmental hurdle before exiting the thymus as mature T cells.
A second selection process, known as negative selection, takes place in the corticomedullary and medulla regions of the thymus.
Negative Selection:
Thymic stromal cells express a variety of self-antigens and present peptide fragments of these antigens to positively selected thymocytes.
A strong reaction between a thymocyte’s TCR and any of the self-peptides presented by the thymic stroma indicates a high potential for autoreactivity.
To prevent such self-reactive T cells from leaving the thymus and subsequently attacking the body’s own cells and tissues, thymocytes with strongly binding TCRs are negatively selected and undergo apoptosis.
Mature T Cells:
Once a T cell exits the thymus, it is considered mature.
Mature T cells that have not yet encountered the specific peptide epitope recognized by their TCR are referred to as "naïve."
To enhance the probability of a mature naïve T cell encountering its specific antigen, these cells spend their lives circulating and recirculating between the bloodstream and lymphatics.
Each naïve T cell forms many contacts with MHC molecules on antigen-presenting cells (APCs) throughout the body.
The journey to activation may last several years for an individual naïve T cell.
Most will recirculate in vain, never uniting with an APC bearing the peptide antigen recognized by their TCR.
T Lymphocyte Activation:
When antigen recognition occurs, T lymphocytes are activated and differentiate into functionally active cells.
Activated CD4+ Th cells immediately begin to proliferate and secrete cytokines.
Signals found within the environment where Th-cell activation occurs, such as cytokines secreted by nearby APCs, influence which cytokines are produced by the Th cells.
CD8+ cytotoxic T cells are also activated by antigen recognition.
Responsive to stimulation by APCs as well as cytokines produced by Th cells, cytotoxic T cells proliferate and begin to seek and destroy cells displaying their specific antigen.
B-Cell Differentiation
B cells are derived from a hematopoietic stem cell that develops into an early lymphocyte progenitor in the bone marrow.
Unlike T cells, which leave the bone marrow and travel to the thymus to mature, B cells mature within the bone marrow itself.
Stromal cells form special niches, promoting the maturation of B-cell precursors.
B-cell precursors go through an ordered developmental process that prepares them for their role in antibody production and, at the same time, restricts the specific antigen to which any one B cell can respond.
The first phase of B-cell development in the bone marrow, which results in mature B cells that have not yet been exposed to antigen, is known as the antigen-independent phase.
This phase can be divided according to the formation of several distinct subpopulations: pro-B cells, pre-B cells, immature B cells, and mature B cells.
For B cells that reach maturity and encounter their specific antigen, an antigen-dependent phase of development begins.
The antigen-dependent phase typically involves the generation of plasma cells and, in many cases, long-lived memory B cells.
Pro-B Cells
At the earliest developmental stage, B-cell progenitors receive signals from bone marrow stromal cells through cell-to-cell contact as well as soluble cytokines, such as IL-7.
This signaling induces the expression of several transcription factors, or proteins that control the expression of genes.
Some important transcription factors expressed within pro-B cells are E2A, early B-cell factor (EBF), interferon regulatory factor 8 (IFR8), and paired box protein 5 (PAX5).
Working in concert, these factors drive the expression of genes that distinguish pro-B cells from progenitor cells.
Such gene expression is required for continued survival and development of the pro-B cell.
One of the most important events of the pro-B cell phase is rearrangement of the B-cell–receptor (BCR) genes.
B-Cell Receptor (BCR)
The BCR is a cell surface version of an immunoglobulin, or antibody molecule.
BCRs share similarities with T-cell receptors.
Composed of two different chains (TCRs consist of α and β chains, whereas BCRs contain heavy and light chains).
Have variable regions, which determine their epitope specificity.
Contain constant regions, which allow for intracellular signaling and activation of the lymphocyte expressing them.
Have similar gene regions (i.e., V, D, and J).
Use similar mechanisms for gene rearrangement.
BCR genes undergo rearrangement in a stepwise manner, beginning with the heavy-chain genes.
Heavy-chain genes contain multiple V, D, and J segments.
Enzymes bring together one V, one D, and one J segment by looping out intervening DNA.
The enzyme terminal deoxynucleotidyl transferase incorporates random nucleotides into the joints between V-D and D-J.
Pro-B cells generate unique BCR genes not found in the genome, and (theoretically) not shared with any other B cell.
The possibility exists that certain BCR heavy-chain rearrangements may shift the gene out of frame or result in a stop codon in the middle of the gene, leading to a nonfunctional heavy chain. Rearrangement continues until a functional heavy chain can be produced using the gene templates for either heavy-chain allele.
Once heavy-chain rearrangement is completed successfully on one chromosome, allelic exclusion silences expression of the heavy-chain gene on the opposite chromosome.
For a pro-B cell to progress to the next phase of differentiation, at least one heavy-chain gene must undergo successful rearrangement.
Pre-B Cells
Once production of successfully rearranged heavy chains begins, developing B cells enter the pre-B-cell stage of differentiation.
Heavy chains accumulate in the cytoplasm.
Heavy chains travel to the cell surface and combine with a surrogate light chain, as well as two shorter chains, Ig-α and Ig-β, to form a structure known as the pre–B-cell receptor (pre-BCR).
Pre-B cells that assemble the pre-BCR undergo several rounds of cell division, resulting in many clones of the original cell, all of which express an identical heavy chain.
Light-chain gene rearrangement begins. Humans possess two different types of light-chain genes: κ and λ.
κ and λ light-chain loci are composed of multiple V and J segments.
During rearrangement, a V segment, J segment, and light- chain constant region are stitched together.
Once successfully rearranged light chains are expressed, macromolecular complexes comprised of two light chains and two heavy chains are formed. These immunoglobulins are fastened together by disulfide bonds and journey to the cell surface to replace the pre-BCR.
Because the heavy chains synthesized during the pre–B-cell stage incorporate the µ constant region, the first class of immunoglobulin produced is immunoglobulin M (IgM).
The appearance of a functional BCR on the B-cell surface signifies entry of the cell into the next phase of development, the immature B cell.
Immunoglobulins
Immunoglobulins consist of two heavy chains and two light chains connected together by disulfide bonds.
The immunoglobulins are grouped into five major classes based on the type of heavy chain they contain: IgM, immunoglobulin G (IgG), immunoglobulin A (IgA), immunoglobulin D (IgD), and immunoglobulin E (IgE).
Each immunoglobulin molecule contains two identical light chains, which are either both κ or both λ.
Immature B Cells
The appearance of a functional IgM BCR on the cell surface indicates that rearrangement of the genes encoding the receptor is now complete, and that a new B cell exists with the potential to produce antibody for a specific and unique epitope.
The immunoglobulin variable regions, found on both the light and heavy chains, determine the antigen specificity of the immature B cell and its IgM BCR.
Because gene rearrangement produces BCRs with random specificities, B cells require a process of negative selection.
Negative Selection in Immature B Cells
Mature B cells respond to binding of antigen to the BCR by activation, proliferation, and antibody production. Immature B cells respond to the same signals by halting their development and undergoing apoptosis.
The majority of B cells capable of producing antibody to self-antigens are deleted before even exiting the bone marrow.
The elimination of B cells that bear self-reactive receptors is known as central tolerance, and it is estimated that more than 90% of B cells die in this manner.
Surface Markers on Immature B Cells
Numerous other surface markers begin to appear during the immature B-cell phase. CD21, CD40, and class II MHC molecules are just some of the proteins and glycoproteins that decorate the external membrane of the immature B cell.
These markers are useful for laboratory identification of B cells and are also essential to the function of B cells—especially their role in antigen presentation to CD4+ Th cells.
CD21 acts as a receptor for a breakdown product of the complement component C3, known as C3d.
The presence of the CD21 receptor enhances the likelihood of contact between B cells and antigens because antigens frequently become coated with complement fragments during the immune response.
CD40 and class II MHC are important for the interaction of B cells with Th cells.
Mature B Cells
A B cell that expresses a functional IgM BCR, survives selection by not reacting to self-antigens, and begins to display certain B-cell markers (CD21, CD40, and MHC) is considered a mature B cell.
B cells that have achieved these milestones exit the bone marrow and are carried in the blood to the spleen for the next stage in their development.
In the spleen, immature B cells develop into one of two types of mature B cells, known as follicular B cells and marginal-zone B cells.
Follicular B cells constantly recirculate between the blood and secondary lymphoid organs in search of their specific antigens, whereas marginal-zone B cells remain in the spleen to respond quickly to blood-borne pathogens.
Both marginal-zone and follicular B cells produce antibody, but the circumstances that trigger antibody production, the types of antibody produced, and the duration of the response are very different between these two populations.
Follicular B Cells
The term follicular refers to the region of the lymph node where this type of mature B cell tends to localize during its movements throughout the body.
Lymphoid follicles represent dense clusters of naïve B cells awaiting exposure to their specific antigens.
When antigen recognition occurs, follicular B cells make contact with CD4+ follicular helper (Tfh) cells.
Cooperation between antigen-activated B cells and Tfh cells is critical for many B-cell processes, including the formation of immunologic memory.
Marginal-Zone B Cells
Marginal-zone B cells receive their name from the anatomical site in which they are most concentrated, the marginal sinus of the spleen.
Most marginal-zone B cells recognize polysaccharide antigens found on common bacterial pathogens.
When marginal-zone B cells contact their specific antigens, they don’t receive help from Tfh cells. Instead, they differentiate into IgM-secreting plasma cells and only stop once the invading microbes have been eliminated.
Marginal-zone response must begin anew upon each exposure to a particular polysaccharide antigen because Tfh cell help is essential for the formation of immunologic memory.
IgM and IgD on Mature B Cells
In addition to an IgM BCR, most mature B cells also express an IgD form of the BCR.
The IgM and IgD BCRs expressed on a particular B cell have the same antigenic specificity.
The presence of both IgM and IgD on the cell membrane signifies a mature B cell.
B-Cell Activation
When a BCR binds its specific antigen, multiple BCR molecules are brought together, initiating an intracellular signaling cascade.
These signals drive the B cell to enter a proliferative stage where it divides rapidly to produce both antibody-secreting plasma cells and, for follicular B cells, memory B cells.
Plasma Cells
Plasma cells express very little immunoglobulin on their surface membranes but have abundant cytoplasmic immunoglobulin.
The oval-shaped nuclei of plasma cells often contain heavily clumped, dark-staining chromatin.
Plasma cells possess ample endoplasmic reticulum and a well-defined Golgi.
Resident plasma cells are a common feature of the bone marrow and the germinal centers found in peripheral lymphoid organs.
Plasma cells survive in bone marrow niches surrounded by stromal cells, which provide stimulation to plasma cells via cytokines; this allows plasma cells to be long-lived and fosters their continual production of antibodies.
Plasma cells located in tissues other than the bone marrow produce antibody for only a short time before dying.
A key surface marker found on plasma cells is CD138.
Role of T Cells in Adaptive Immune Response
When infection occurs in the body’s tissues, APCs such as macrophages and dendritic cells are among the first immune cells to respond.
APCs engulf pathogens at these distal sites of infection and carry associated antigens to local lymph nodes.
Upon arrival at lymph nodes near the site of infection, antigen-laden APCs encounter naïve T cells in the process of patrolling for antigen.
The continuous recirculation of naïve T cells between the blood and lymph nodes greatly increases the likelihood of an APC connecting with one or more of the few T cells whose TCRs recognize the antigens carried by the APC.
Antigen Presentation
The primary mode of communication between T cells and APCs involves direct cell-to-cell contact.
Using an immunologic process known as antigen presentation, APCs display peptide antigens to T cells via major histocompatibility molecules (MHC, also called human leukocyte antigen [HLA]).
MHC molecules cradle antigenic peptides in a manner similar to a bun holding a hot dog and allow the TCR to bind along the entire length of the peptide.
Humans and related mammals express two different forms of MHC protein, MHC class I and class II.
Class I MHC molecules present peptide antigens derived from cytoplasmic sources. Class I MHC molecules present antigen to cytotoxic T cells.
Class II MHC molecules present peptides captured from the extracellular space. Class II MHC molecules allow APCs to present such extracellular-derived peptide antigens to Th cells.
CD4 and CD8 Roles
The interaction between the cytotoxic TCR and class I MHC is stabilized by CD8, a reliable marker for cytotoxic T cells.
The interaction between class II MHC and the Th TCR is stabilized by CD4. Thus, CD4 is commonly used as a marker to identify Th cells in the laboratory.
T-Cell Activation
If the TCR recognizes one of the many antigens being presented by an APC, an intracellular signaling cascade is initiated within the T cell.
TCR signaling alone is not sufficient to activate a naïve T cell. For activation to occur, the APC must also provide costimulation to the T cell by expressing CD80 or CD86, molecules that ligate the T-cell surface protein CD28.
The combination of signals that arises when the TCR recognizes its specific peptide and CD28 is ligated transforms a naïve T cell into an activated T cell.
T Helper (Th) Cells
Th cells are not phagocytic, cannot kill infected cells, and are incapable of the production and secretion of antibodies.
Th cells drive the activities of other immune cells that act directly to fight infection (macrophages, cytotoxic T cells, and B cells).
When activated by APCs, Th cells travel to infected tissues to orchestrate the immune response via the secretion of cytokines.
Several subsets of activated Th cells exist, of which the most prominent are termed Th1, Th2, and Th17 cells, each of which has a different role in immune responses
The ability of newly activated Th cells to differentiate into these various subsets is influenced by the cytokines present during activation.
Each Th subset, in turn, produces a unique set of cytokines capable of driving the immune response to target a particular type of infection.
Th1 cells produce interferon-gamma (IFN-γ), interleukin-2 (IL-2), and tumor necrosis factor-α (TNF-α), cytokines that activate CD8+ cytotoxic lymphocytes and macrophages to fight intracellular parasites.
Th2 cells produce a variety of cytokines, including interleukins (IL) IL-4, IL-5, IL-6, IL-9, IL-10, and IL-13. The essential role of Th2 cells is to control the clearance of extracellular parasites, such as intestinal worms. Th2 cells are also thought to play a role in allergy.
Th17 cells produce the cytokines IL-17 and IL-22, which lead to the recruitment of granulocytes in response to an extracellular bacterial infection. Granulocyte activity can sometimes cause immune-mediated damage. Because of this, the Th17 response is often associated with pathology.
T regulatory (Treg) cells, possess the CD4 antigen as well as CD25. These cells comprise approximately 5% of all CD4+ T cells and play an important role in suppressing the immune response to self-antigens and harmless antigens, such as those in common foods. They inhibit the proliferation of other T-cell populations by secreting inhibitory cytokines and possess TCRs that recognize antigenic peptide in MHC class II; the response of Tregs is antigen-specific.
T Follicular Helper (Tfh) Cells
To assist B cells in antibody production, a special subpopulation of Th cells, known as T follicular helper (Tfh) cells, remains in the lymph nodes and interacts with B cells and plasma cells there.
Tfh cells provide essential signaling to B cells as they undergo processes such as activation, immunoglobulin class switching, affinity maturation, and the formation of B- cell memory.
Memory T Cells
During the many rounds of cell division that follow Th-cell activation, two distinct populations of cells are formed. Most Th cells begin to secrete cytokines and may travel to infected tissues where their activities are most needed.
A small percentage of the Th cells generated after activation will differentiate into memory cells; memory Th cells enter a quiescent state and await re-exposure to their specific antigen. If/when contact with antigen occurs again, memory cells respond rapidly by re-entering cell division and immediately secreting appropriate cytokines.
Action of Cytotoxic T Cells
CD8-expressing cytotoxic T cells (Tc), also known as cytotoxic T lymphocytes (CTLs), play a different role than Th cells.
Activated cytotoxic T cells migrate to sites of infection and initiate apoptosis in cells infected with intracellular parasites and viruses.
Unlike NK cells, which fulfill a similar role but recognize infection through germline-encoded receptors, the activity of cytotoxic T cells is driven by TCR recognition and is therefore antigen-specific.
Cytotoxic T Cell Mechanisms
Upon recognition of peptide antigen in class I MHC on the surface of a target cell, cytotoxic T cells kill their targets:
Release of cytotoxic granules from the T-cell cytoplasm
Ligation of death receptors on a target cell’s surface
The target cell rapidly undergoes apoptosis.
Cytotoxic Granules
Contain perforins and granzymes.
Perforins are proteins that insert into target-cell membranes and polymerize to form pores.
Granzymes are serine proteases that can initiate the fragmentation of DNA in the target cell.
TCR recognition of antigen by a cytotoxic T cell causes accumulation of granules in the T-cell cytoplasm adjacent to the target cell; the granules are then released by the T cell in the direction of the target cell.
Perforin begins forming holes in the target-cell membrane, through which granzymes can enter.
Once in the target cell, granzymes activate nuclease enzymes that cleave DNA and disrupt mitochondria, leading to apoptosis.
Ligation of Death Receptors
TCR recognition of antigen complexed with target-cell MHC leads to expression of the death-inducing protein Fas-ligand (FasL) by the T cell.
When FasL binds to Fas on the target-cell membrane, apoptotic pathways similar to those activated by the granzymes are set in motion.
Role of B Cells in Adaptive Immunity
Exposure of the IgM BCR to its specific antigenic epitope is the essential first step in the activation of a mature B cell.
Antigen recognition by the different types of mature B cells (follicular and marginal zone) occurs in different anatomical regions—the lymph nodes and the spleen, respectively.
T-Dependent and T-Independent Antigens
Because the follicular B-cell response depends heavily on the activity of Tfh cells to promote an effective antibody response, antigens that provoke this type of response are often referred to as T- dependent antigens.
Marginal-zone B cells don’t require the help of Tfh cells; the antigens recognized by marginal-zone B cells are referred to as T-independent antigens.
Immune Response to T-Dependent Antigens
T-dependent antigens are almost always proteins, because proteins are the only type of antigen that can stimulate a T-cell response.
To reach the lymph nodes, protein antigens may travel suspended or dissolved within lymphatic fluid, or they may be carried by macrophages or dendritic cells.
Protein antigens arrive at the B-cell–rich follicles in their native state (that is, not processed or denatured).
BCRs bind to antigenic epitopes exposed on an antigen’s surface.
Follicular B-Cell Activation
BCR antigen recognition initiates a cascade of intracellular signaling within the B cell, driving the cell into an activated state.
In response to BCR signaling, the B-cell cytoskeleton is mobilized to internalize the bound antigen using a process known as endocytosis; this allows for digestion of the antigen and presentation of its constituent peptides on class II MHC molecules on the B-cell surface.
Following activation, B cells migrate to the edges of follicles, where they begin to interact with Tfh cells; B cells act as APCs, presenting peptide fragments derived from internalized antigen to Tfh cells.
If TCR binding to one of these antigenic peptides occurs, a T cell–B cell pair is formed, and the T-dependent phase of the B-cell response begins.
Tfh Cell Signaling to B Cells
Tfh cells provide activated B cells with two additional signals that contribute to the B-cell response.
The first signal provided by the T cell requires physical contact between T cells and B cells; TCR recognition of peptide antigen causes Tfh cells to express CD40 ligand (CD40L), which binds to CD40 on B cells.
T cells also signal to B cells through the secretion of cytokines; T cells secrete IL-2, which binds to CD25 expressed on B cells and spurs them to enter a phase of rapid cell division.
Daughter B-Cell Fates
Daughter B cells produced during the proliferative phase of the T-dependent response have two potential fates:
Some remain in contact with T cells and differentiate into IgM-secreting plasma cells.
Others form germinal centers within follicles and participate in a series of processes that enhance the antibody response through time.
Germinal Center Reaction
Requires interaction with T cells/affiliation of CD40 with CD40L/secretion of cytokines.
Involves three overlapping processes: immunoglobulin isotype switching, affinity maturation, and memory-cell generation.
Isotype Switching
IgM is the most common class of immunoglobulin molecule incorporated into the BCRs of marginal-zone B cells and follicular B cells before/at very early times after antigen recognition.
B cells can express other classes of immunoglobulin through precisely controlled rearrangement of the heavy-chain genes: IgG, IgA, and IgE.
Under the direction of T cells, germinal center B cells can change which class of antibody they express. The gene recombination that occurs during isotype switching is specific to the constant region of the heavy
T Helper (Th) Cells, T-Independent Antigens, T Regulatory (Treg) Cells, Thymocytes
Innate Immunity:
Includes barrier defenses (e.g., skin, mucous membranes), phagocytic cells (macrophages, neutrophils, dendritic cells), and preformed soluble factors (complement proteins, acute phase proteins, cytokines like interferons).
Responses are general, with entire pathogen classes (e.g., bacteria, viruses, fungi) eliciting similar responses via pattern recognition receptors (PRRs) like Toll-like receptors (TLRs).
Responses are unchanging over time and lack immunological memory. Subsequent encounters with the same pathogen evoke the same response.
Inflammation is a key component, involving vasodilation, increased vascular permeability, and immune cell recruitment.
Adaptive Immunity:
Characterized by specificity for individual microbes and pathogens, mediated by T and B lymphocytes.
Exhibits memory of prior antigen exposures, leading to faster and more effective responses upon re-exposure.
Responses enhance upon repeated exposure (affinity maturation and increased antibody production).
Represents a more tailored, pathogen-specific response compared to innate immunity, targeting unique epitopes on antigens.
Requires time (several days to weeks) after initial exposure to mount a response due to clonal expansion and differentiation of lymphocytes.
Can provide lifelong immunity against reinfection, mediated by long-lived memory cells.
Divided into cell-mediated immunity (T cells) and humoral immunity (antibodies by B cells).
Key players include antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells, which process and present antigens to T cells.
T Lymphocytes (T cells):
Responsible for cell-mediated adaptive immunity, recognizing antigens presented by MHC molecules on APCs.
Divided into cytotoxic T cells (CD8+ T cells) and helper T (Th) cells (CD4+ T cells).
Cytotoxic T cells (Tc): Kill target cells infected with intracellular pathogens (e.g., viruses, bacteria, parasites) and cancerous cells by releasing cytotoxic granules containing perforin and granzymes, or by engaging death receptors like Fas.
Helper T (Th) cells: Control the immune response by secreting cytokines (signaling molecules for communication between immune cells), coordinating the activities of other immune cells like B cells, macrophages, and cytotoxic T cells.
Subpopulations exist based on cytokine secretion profiles (Th1, Th2, Th17, Treg, Tfh), each promoting different types of immune responses.
B Lymphocytes (B cells):
Produce antibodies (immunoglobulins) that recognize and bind to specific antigens.
Antibody roles include:
Labeling targets for phagocytosis (opsonization).
Neutralizing viruses and toxins by blocking their ability to infect cells.
Blocking microbe adhesion to tissues, preventing colonization.
Activating the complement system, leading to pathogen lysis.
Antibody classes (IgM, IgG, IgA, IgE, IgD) have different functions and tissue distributions.
Lymphocyte Receptor Gene Rearrangement:
A key difference between adaptive immunity lymphocytes and innate immunity cells, allowing for a vast repertoire of antigen specificities.
Genes coding for lymphocyte receptors (TCRs and BCRs) undergo somatic recombination during T-cell or B-cell development.
Each lymphocyte is equipped to respond to a unique epitope associated with a specific pathogen, ensuring broad coverage against diverse threats.
V(D)J recombination involves the random selection and joining of variable (V), diversity (D), and joining (J) gene segments, creating a highly diverse set of receptors.
Imprecise joining and insertion of additional nucleotides further enhance receptor diversity.
Adaptive Immune System Diversity:
The adaptive immune system represents an immune army capable of combating any conceivable infectious threat due to slightly different receptor specificities on each lymphocyte.
This diversity is generated through random gene rearrangement, ensuring that there are lymphocytes capable of recognizing virtually any antigen.
Clonal Expansion:
Only lymphocytes responsive to epitopes on invading pathogens are activated and proliferate in a process called clonal expansion.
Proliferating lymphocytes give rise to populations of cells with genetically identical receptors specific for the same epitope, amplifying the immune response.
Clonal expansion requires co-stimulatory signals and cytokine support to ensure effective activation and differentiation of lymphocytes.
Delay in Adaptive Response:
The characteristic delay between initial infection and an effective adaptive response arises because pathogen-responsive lymphocytes undergo massive clonal expansion to reach the necessary number of effector cells.
This delay is also due to the time required for antigen processing and presentation, lymphocyte migration, and differentiation into effector cells.
Effector Cells vs. Memory Cells:
Most progeny from clonal expansion are effector cells (cytokine-secreting/cytotoxic T cells or antibody-producing B cells) that act immediately to eliminate the pathogen.
A small proportion become memory cells, which enter a quiescent state and are long-lived, providing long-term immunity.
Memory cells activate rapidly upon re-encountering their specific epitope, resulting in a swifter, greater magnitude response than the primary response (immunological memory).
Memory cells can be divided into central memory cells (Tcm) and effector memory cells (Tem), with different homing properties and activation thresholds.
Lymphocyte Production (Differentiation):
Begins early in fetal development, ensuring the establishment of a functional immune system at birth.
Progenitors of T and B cells appear in the fetal liver as early as 8 weeks of pregnancy.
Bone marrow becomes the primary lymphocyte source later in fetal development and throughout adult life, providing a continuous supply of new lymphocytes.
Lymphoid Progenitor Cells:
The earliest precursors of T and B lymphocytes are generated by asymmetric division of lymphoid progenitor cells in the bone marrow.
Asymmetry refers to one daughter cell taking on lymphocyte characteristics while the other retains stem cell-like plasticity, ensuring a sustained supply of lymphoid progenitors.
These progenitors express specific markers like CD34 and CD10, allowing for their identification and isolation.
T-Cell Differentiation:
T-cell precursors leave the bone marrow and migrate to the thymus, where they undergo maturation and selection.
The thymus is divided into the outer cortex and inner medulla, each providing a distinct microenvironment for T-cell development.
T-cell precursors become known as thymocytes once in the thymus.
Thymocytes enter at the cortico-medullary junction and migrate toward the thymic cortex, undergoing a series of developmental stages
During their 3-week migration, differentiation processes occur:
Rearrangement of T-cell receptor (TCR) genes, generating a diverse repertoire of antigen specificities.
Changes in expression of thymocyte cell surface markers (CD4, CD8), defining their functional potential.
Selection of thymocytes with functional receptors that can recognize MHC molecules.
Deletion of thymocytes with self-reactive potential, preventing autoimmunity.
Interactions with stromal cells (macrophages, dendritic cells, fibroblasts, thymic epithelial cells) are critical for these processes, providing essential signals for survival, proliferation, and differentiation.
Major stages of T-cell development: double-negative (DN), double-positive (DP), and mature T cells, defined by their expression of CD4 and CD8 markers.
CD Markers
Key cluster of differentiation (CD) markers are found on the surface of immune cells, including lymphocytes, serving as identification tags and functional molecules.
CD markers provide a fingerprint, allowing classification of T and B cells and their developmental stage, as well as their activation state and function.
Bound by fluorescent-tagged antibody reagents; the number of lymphocytes positive for each marker can be determined using flow cytometry, a powerful technique for analyzing immune cell populations.
Routinely used in the diagnosis of leukemias, lymphomas, and immunodeficiency diseases such as AIDS, as well as for monitoring immune responses in transplantation and autoimmune diseases.
Examples of CD markers include CD3 (TCR complex), CD4 (Th cells), CD8 (cytotoxic T cells), CD19 (B cells), CD56 (NK cells), and CD45 (all leukocytes).
Double-Negative (DN) Stage:
Cytotoxic T cells (Tc) and Th cells are identified by differential expression of CD4 and CD8 markers.
CD4 identifies Th cells, which help coordinate immune responses, while CD8 identifies cytotoxic T cells, which kill infected cells.
Early thymocytes lack both CD4 and CD8, thus known as double-negative (DN) thymocytes.
DN thymocytes aggregate in the outer cortex, proliferating under the influence of cytokines like interleukin-7 (IL-7), which promotes their survival and development.
DN stage is further divided into DN1, DN2, DN3, and DN4 based on the expression of CD44 and CD25 markers, reflecting distinct developmental steps.
TCR Gene Rearrangement (DN Stage):
Rearrangement of genes coding for the TCR begins during the DN stage, specifically at the DN2 and DN3 stages.
Random shuffling and altering of TCR genes yields expression of a unique antigen receptor by each thymocyte, creating a highly diverse T-cell population capable of recognizing a wide range of antigens.
This process involves V(D)J recombination, mediated by recombinase enzymes RAG1 and RAG2.
TCR Structure:
The TCR comprises two transmembrane proteins: alpha (α) and beta (β) chains, which associate with the CD3 complex for signaling.
Each chain possesses an intracellular signaling domain, a membrane-spanning domain, and an extracellular domain tipped with a variable region responsible for antigen recognition.
Variable regions of α and β chains are responsible for antigen recognition and undergo rearrangement to generate diversity.
The TCR recognizes peptide antigens presented by MHC molecules on APCs.
TCR Gene Organization:
Before rearrangement, TCR β chain genes (chromosome 7) are divided into V, D, and J sections, each with multiple segments, providing a template for generating diverse TCRs.
Enzymes in DN thymocytes clip genomic DNA and stitch it back together, randomly aligning one V, one D, one J segment, and constant region segment in a process called V(D)J recombination.
Imprecise cleavage and ligation enhance TCR diversity but may result in nonfunctional TCRs, necessitating quality control mechanisms.
RAG1 and RAG2 enzymes mediate the cutting and joining of DNA segments during V(D)J recombination.
Strategies for Functional TCR β Chain:
T cells are diploid, so β chain genes on both copies of chromosome 7 simultaneously undergo rearrangement, doubling the probability of a functional β chain.
Enzymes mediating TCR gene rearrangement continue clipping and stitching variable-region DNA until a functional TCR is generated, ensuring that each T cell expresses a functional receptor.
This process is tightly regulated to prevent excessive DNA damage and maintain genomic stability.
Pre-TCR Formation:
To test functionality of TCR β chains, a surrogate α chain (no variable region), called pre-Tα, is temporarily expressed, forming a pre-TCR with the rearranged β chain.
If the DN thymocyte expresses a functional β chain, the pre-TCR provides survival signals, promoting further development and preventing apoptosis.
This checkpoint ensures that only thymocytes with functional TCR β chains proceed to the next stage of development.
Gamma-Delta (γδ) T Cells:
Early on, some thymocytes rearrange and express gamma (γ) and delta (δ) TCR chains when there isn't a productive rearrangement of DNA coding for a β chain.
This unique population (10% or less) is γδ T cells, which have distinct functional properties compared to αβ T cells.
γδ T cells proceed down an alternate developmental pathway, typically remaining negative for both CD4 and CD8.
Dominant T-cell population in the skin, intestinal epithelium, and pulmonary epithelium, providing rapid immune responses in these tissues.
Tasks include wound healing and protection of the epithelium, as well as recognition of non-peptide antigens.
Do not require antigen presentation by major histocompatibility complex (MHC) proteins, allowing for faster responses.
Represent an important bridge between innate and adaptive immunity, responding to stress signals and infections.
Double-Positive (DP) Stage:
Successful pre-TCR signaling due to a functioning TCR β chain initiates rearrangement of the TCR α chain, ensuring the formation of a complete TCR.
α chain gene variable regions are divided into V and J sections.
α chain rearrangement is mechanistically identical to β chain rearrangement, involving V(D)J recombination.
The appearance of a functional α chain suppresses further TCR gene rearrangements, ensuring allelic exclusion.
Thymocytes become CD4-positive (CD4+) and CD8-positive (CD8+), known as double-positive (DP) thymocytes.
DP thymocytes express both CD4 and CD8 co-receptors, which interact with MHC molecules on APCs.
Allelic Exclusion:
Thymocytes are diploid cells with the potential to express two different α and β chains.
However, each T cell recognizes only one antigenic peptide, ensuring specificity and preventing autoimmunity.
Once a functional version of the β and α chains is produced, expression of the corresponding TCR gene on the opposite chromosome is permanently shut off, maintaining allelic exclusion.
There are molecular mechanisms involving feedback inhibition and epigenetic modification to make sure there is allelic exclusion.
CD3/TCR Complex:
The TCR α and β chains occur in a complex with six other molecules common to all T cells, known collectively as CD3/TCR complex.
The six chains of the nonspecific CD3 portion of the complex assist in intracellular signaling when an antigen binds to the TCR, transducing the signal into the cell.
These chains occur in three pairs: delta-epsilon (-), gamma-epsilon (-), and a tau-tau (-) chain, each playing a role in signal transduction.
The CD3 complex is essential for T-cell activation and function.
MHC Restriction:
T-cell receptors do not recognize peptide antigen alone. Peptide antigen must be held in an MHC molecule, or “presented” to T cells, ensuring that T cells respond to processed antigens.
This property of T cells is established during the DP stage of thymocyte development, shaping their antigen recognition specificity.
MHC restriction ensures that T cells recognize antigens derived from within cells (MHC class I) or from extracellular sources (MHC class II).
Positive Selection:
Gene rearrangement produces TCRs with nearly unlimited specificities; each receptor must be tested for its ability to interact with MHC, ensuring that only functional TCRs are selected.
Thymocytes encounter stromal cells in the thymic cortex that express MHC class I and II proteins, presenting self-peptides.
Interactions between the DP thymocyte’s TCR and stromal MHC determine the thymocyte’s fate:
TCRs that bind with very high affinity to MHC, or those that fail to bind to MHC at all, are induced to die by apoptosis (death by neglect).
Only those DP thymocytes with receptors that bind moderately to MHC survive, ensuring they have the potential to recognize foreign antigens.
T cells with receptors that bind to MHC too strongly have a high potential to react with self-antigens, leading to autoimmunity.
TCRs that don’t bind to MHC can’t function as mature T cells, as they cannot be activated by antigen presentation.
Positive selection ensures MHC restriction, shaping the T-cell repertoire.
CD4 and CD8 Role in MHC Restriction:
Two important molecules for establishing the MHC restriction of T cells are CD4 and CD8, which bind to MHC class I and class II molecules, respectively.
During the DP stage, thymocytes express both CD4 and CD8.
Depending on which class of MHC molecule a positively selected thymocyte recognizes, the expression of the opposite marker begins to decrease substantially, committing the thymocyte to either the helper or cytotoxic lineage.
Thymocytes possessing TCRs that recognize MHC-II express CD4, whereas those that bind to MHC-I express CD8.
Stable expression of either CD4 or CD8 and loss of expression of the opposite marker signify the entrance of the thymocyte into the next phase of differentiation, the “single-positive” (SP) stage.
Single-Positive (SP) Stage:
Thymocytes that survive positive selection must overcome one final developmental hurdle before exiting the thymus as mature T cells: negative selection.
A second selection process, known as negative selection, takes place in the corticomedullary and medulla regions of the thymus.
Negative Selection:
Thymic stromal cells express a variety of self-antigens and present peptide fragments of these antigens to positively selected thymocytes.
A strong reaction between a thymocyte’s TCR and any of the self-peptides presented by the thymic stroma indicates a high potential for autoreactivity.
To prevent such self-reactive T cells from leaving the thymus and subsequently attacking the body’s own cells and tissues, thymocytes with strongly binding TCRs are negatively selected and undergo apoptosis (clonal deletion).
AIRE (autoimmune regulator) is a transcription factor that promotes the expression of tissue-specific antigens in the thymus, facilitating negative selection.
Mature T Cells:
Once a T cell exits the thymus, it is considered mature and ready to patrol for foreign antigens.
Mature T cells that have not yet encountered the specific peptide epitope recognized by their TCR are referred to as "naïve."
To enhance the probability of a mature naïve T cell encountering its specific antigen, these cells spend their lives circulating and recirculating between the bloodstream and lymphatics.
Each naïve T cell forms many contacts with MHC molecules on antigen-presenting cells (APCs) throughout the body, increasing the chances of antigen recognition.
The journey to activation may last several years for an individual naïve T cell.
Most will recirculate in vain, never uniting with an APC bearing the peptide antigen recognized by their TCR.
T Lymphocyte Activation:
When antigen recognition occurs, T lymphocytes are activated and differentiate into functionally active cells, initiating an immune response.
Activated CD4+ Th cells immediately begin to proliferate and secrete cytokines, coordinating the activities of other immune cells.
Signals found within the environment where Th-cell activation occurs, such as cytokines secreted by nearby APCs, influence which cytokines are produced by the Th cells, shaping the immune response.
CD8+ cytotoxic T cells are also activated by antigen recognition, enabling them to kill infected cells.
Responsive to stimulation by APCs as well as cytokines produced by Th cells, cytotoxic T cells proliferate and begin to seek and destroy cells displaying their specific antigen.
B-Cell Differentiation
B cells are derived from a hematopoietic stem cell that develops into an early lymphocyte progenitor in the bone marrow.
Unlike T cells, which leave the bone marrow and travel to the thymus to mature, B cells mature within the bone marrow itself.
Stromal cells form special niches, promoting the maturation of B-cell precursors by providing essential signals and growth factors.
B-cell precursors go through an ordered developmental process that prepares them for their role in antibody production and, at the same time, restricts the specific antigen to which any one B cell can respond.
The first phase of B-cell development in the bone marrow, which results in mature B cells that have not yet been exposed to antigen, is known as the antigen-independent phase.
This phase can be divided according to the formation of several distinct subpopulations: pro-B cells, pre-B cells, immature B cells, and mature B cells.
For B cells that reach maturity and encounter their specific antigen, an antigen-dependent phase of development begins.
The antigen-dependent phase typically involves the generation of plasma cells and, in many cases, long-lived memory B cells, which provide long-term immunity.
Pro-B Cells
At the earliest developmental stage, B-cell progenitors receive signals from bone marrow stromal cells through cell-to-cell contact as well as soluble cytokines, such as IL-7, which promotes their survival and proliferation.
This signaling induces the expression of several transcription factors, or proteins that control the expression of genes.
Some important transcription factors expressed within pro-B cells are E2A, early B-cell factor (EBF), interferon regulatory factor 8 (IFR8), and paired box protein 5 (PAX5).
Working in concert, these factors drive the expression of genes that distinguish pro-B cells from progenitor cells, committing them to the B-cell lineage.
Such gene expression is required for continued survival and development of the pro-B cell.
One of the most important events of the pro-B cell phase is rearrangement of the B-cell–receptor (BCR) genes, initiating the formation of a functional BCR.
B-Cell Receptor (BCR)
The BCR is a cell surface version of an immunoglobulin, or antibody molecule, that recognizes and binds to specific antigens.
BCRs share similarities with T-cell receptors.
Composed of two different chains (TCRs consist of α and β chains, whereas BCRs contain heavy and light chains).
Have variable regions, which determine their epitope specificity, allowing them to recognize a wide range of antigens.
Contain constant regions, which allow for intracellular signaling and activation of the lymphocyte expressing them.
Have similar gene regions (i.e., V, D, and J).
Use similar mechanisms for gene rearrangement, involving V(D)J recombination.
BCR genes undergo rearrangement in a stepwise manner, beginning with the heavy-chain genes.
Heavy-chain genes contain multiple V, D, and J segments.
Enzymes bring together one V, one D, and one J segment by looping out intervening DNA, creating a diverse repertoire of heavy chains.
The enzyme terminal deoxynucleotidyl transferase incorporates random nucleotides into the joints between V-D and D-J, further increasing diversity.
Pro-B cells generate unique BCR genes not found in the genome, and (theoretically) not shared with any other B cell.
The possibility exists that certain BCR heavy-chain rearrangements may shift the gene out of frame or result in a stop codon in the middle of the gene, leading to a nonfunctional heavy chain. Rearrangement continues until a functional heavy chain can be produced using the gene templates for either heavy-chain allele.
Once heavy-chain rearrangement is completed successfully on one chromosome, allelic exclusion silences expression of the heavy-chain gene on the opposite chromosome, ensuring that each B cell expresses a single BCR specificity.
For a pro-B cell to progress to the next phase of differentiation, at least one heavy-chain gene must undergo successful rearrangement.
Pre-B Cells
Once production of successfully rearranged heavy chains begins, developing B cells enter the pre-B-cell stage of differentiation.
Heavy chains accumulate in the cytoplasm.
Heavy chains travel to the cell surface and combine with a surrogate light chain, as well as two shorter chains, Ig-α and Ig-β, to form a structure known as the pre–B-cell receptor (pre-BCR).
Pre-B cells that assemble the pre-BCR undergo several rounds of cell division, resulting in many clones of the original cell, all of which express an identical heavy chain.
Light-chain gene rearrangement begins. Humans possess two different types of light-chain genes: κ and λ.
κ and λ light-chain loci are composed of multiple V and J segments.
During rearrangement, a V segment, J segment, and light- chain constant region are stitched together, forming a complete light chain.
Once successfully rearranged light chains are expressed, macromolecular complexes comprised of two light chains and two heavy chains are formed. These immunoglobulins are fastened together by disulfide bonds and journey to the cell surface to replace the pre-BCR.
Because the heavy chains synthesized during the pre–B-cell stage incorporate the µ constant region, the first class of immunoglobulin produced is immunoglobulin M (IgM).
The appearance of a functional BCR on the B-cell surface signifies entry of the cell into the next phase of development, the immature B cell.
Immunoglobulins
Immunoglobulins consist of two heavy chains and two light chains connected together by disulfide bonds, forming a Y-shaped molecule.
The immunoglobulins are grouped into five major classes based on the type of heavy chain they contain: IgM, immunoglobulin G (IgG), immunoglobulin A (IgA), immunoglobulin D (IgD), and immunoglobulin E (IgE), each with distinct functions and properties.
Each immunoglobulin molecule contains two identical light chains, which are either both κ or both λ, contributing to antigen-binding specificity.
Immature B Cells
The appearance of a functional IgM BCR on the cell surface indicates that rearrangement of the genes encoding the receptor is now complete, and that a new B cell exists with the potential to produce antibody for a specific and unique epitope.
The immunoglobulin variable regions, found on both the light and heavy chains, determine the antigen specificity of the immature B cell and its IgM BCR.
Because gene rearrangement produces BCRs with random specificities, B cells require a process of negative selection to eliminate self-reactive B cells.
Negative Selection in Immature B Cells
Mature B cells respond to binding of antigen to the BCR by activation, proliferation, and antibody production. Immature B cells respond to the same signals by halting their development and undergoing apoptosis, preventing autoimmunity.
The majority of B cells capable of producing antibody to self-antigens are deleted before even exiting the bone marrow, ensuring central tolerance.
The elimination of B cells that bear self-reactive receptors is known as central tolerance, and it is estimated that more than 90% of B cells die in this manner.
Receptor editing, in which self-reactive B cells can rearrange their light-chain genes to change their BCR specificity, can also occur during negative selection.
Surface Markers on Immature B Cells
Numerous other surface markers begin to appear during the immature B-cell phase. CD21, CD40, and class II MHC molecules are just some of the proteins and glycoproteins that decorate the external membrane of the immature B cell.
These markers are useful for laboratory identification of B cells and are also essential to the function of B cells—especially their role in antigen presentation to CD4+ Th cells.
CD21 acts as a receptor for a breakdown product of the complement component C3, known as C3d.
The presence of the CD21 receptor enhances the likelihood of contact between B cells and antigens because antigens frequently become coated with complement fragments during the immune response.
CD40 and class II MHC are important for the interaction of B cells with Th cells, providing co-stimulatory signals for activation.
Mature B Cells
A B cell that expresses a functional IgM BCR, survives selection by not reacting to self-antigens, and begins to display certain B-cell markers (CD21, CD40, and MHC) is considered a mature B cell.
B cells that have achieved these milestones exit the bone marrow and are carried in the blood to the spleen for the next stage in their development.
In the spleen, immature B cells develop into one of two types of mature B cells, known as follicular B cells and marginal-zone B cells.
Follicular B cells constantly recirculate between the blood and secondary lymphoid organs in search of their specific antigens, whereas marginal-zone B cells remain in the spleen to respond quickly to blood-borne pathogens.
Both marginal-zone and follicular B cells produce antibody, but the circumstances that trigger antibody production, the types of antibody produced, and the duration of the response are very different between these two populations.
Follicular B Cells
The term follicular refers to the region of the lymph node where this type of mature B cell tends to localize during its movements throughout the body.
Lymphoid follicles represent dense clusters of naïve B cells awaiting exposure to their specific antigens, providing an environment for B-cell activation and differentiation.
When antigen recognition occurs, follicular B cells make contact with CD4+ follicular helper (Tfh) cells, which provide essential help for antibody production.
Cooperation between antigen-activated B cells and Tfh cells is critical for many B-cell processes, including the formation of immunologic memory, isotype switching, and affinity maturation.
Marginal-Zone B Cells
Marginal-zone B cells receive their name from the anatomical site in which they are most concentrated, the marginal sinus of the spleen.
Most marginal-zone B cells recognize polysaccharide antigens found on common bacterial pathogens, providing rapid protection against encapsulated bacteria.
When marginal-zone B cells contact their specific antigens, they don’t receive help from Tfh cells. Instead, they differentiate into IgM-secreting plasma cells and only stop once the invading microbes have been eliminated.
Marginal-zone response must begin anew upon each exposure to a particular polysaccharide antigen because Tfh cell help is essential for the formation of immunologic memory.
IgM and IgD on Mature B Cells
In addition to an IgM BCR, most mature B cells also express an IgD form of the BCR, further diversifying their antigen recognition capabilities.
The IgM and IgD BCRs expressed on a particular B cell have the same antigenic specificity, ensuring that the B cell responds to the same antigen regardless of which BCR is engaged.
The presence of both IgM and IgD on the cell membrane signifies a mature B cell, ready to respond to antigen stimulation.
B-Cell Activation
When a BCR binds its specific antigen, multiple BCR molecules are brought together, initiating an intracellular signaling cascade, leading to B-cell activation and proliferation.
These signals drive the B cell to enter a proliferative stage where it divides rapidly to produce both antibody-secreting plasma cells and, for follicular B cells, memory B cells.
Co-stimulatory signals from Tfh cells are essential for optimal B-cell activation and differentiation.
Plasma Cells
Plasma cells express very little immunoglobulin on their surface membranes but have abundant cytoplasmic immunoglobulin, reflecting their primary function of antibody production.
The oval-shaped nuclei of plasma cells often contain heavily clumped, dark-staining chromatin, a characteristic histological feature.
Plasma cells possess ample endoplasmic reticulum and a well-defined Golgi, reflecting their high rate of protein synthesis and secretion.
Resident plasma cells are a common feature of the bone marrow and the germinal centers found in peripheral lymphoid organs, providing long-term antibody production.
Plasma cells survive in bone marrow niches surrounded by stromal cells, which provide stimulation to plasma cells via cytokines; this allows plasma cells to be long-lived and fosters their continual production of antibodies.
Plasma cells located in tissues other than the bone marrow produce antibody for only a short time before dying.
A key surface marker found on plasma cells is CD138, also known as syndecan-1, which is involved in cell adhesion and migration.
Role of T Cells in Adaptive Immune Response
When infection occurs in the body’s tissues, APCs such as macrophages and dendritic cells are among the first immune cells to respond, initiating the adaptive immune response.
APCs engulf pathogens at these distal sites of infection and carry associated antigens to local lymph nodes, where they encounter T cells.
Upon arrival at lymph nodes near the site of infection, antigen-laden APCs encounter naïve T cells in the process of patrolling for antigen.
The continuous recirculation of naïve T cells between the blood and lymph nodes greatly increases the likelihood of an APC connecting with one or more of the few T cells whose TCRs recognize the antigens carried by the APC.
Antigen Presentation
The primary mode of communication between T cells and APCs involves direct cell-to-cell contact, allowing for efficient antigen recognition and T-cell activation.
Using an immunologic process known as antigen presentation, APCs display peptide antigens to T cells via major histocompatibility molecules (MHC, also called human leukocyte antigen [HLA]).
MHC molecules cradle antigenic peptides in a manner similar to a bun holding a hot dog and allow the TCR to bind along the entire length of the peptide, maximizing the interaction.
Humans and related mammals express two different forms of MHC protein, MHC class I and class II.
Class I MHC molecules present peptide antigens derived from cytoplasmic sources. Class I MHC molecules present antigen to cytotoxic T cells.
Class II MHC molecules present peptides captured from the extracellular space. Class II MHC molecules allow APCs to present such extracellular-derived peptide antigens to Th cells.
CD4 and CD8 Roles
The interaction between the cytotoxic TCR and class I MHC is stabilized by CD8, a reliable marker for cytotoxic T cells, enhancing antigen recognition and T-cell activation.
The interaction between class II MHC and the Th TCR is stabilized by CD4. Thus, CD4 is commonly used as a marker to identify Th cells in the laboratory.
T-Cell Activation
If the TCR recognizes one of the many antigens being presented by an APC, an intracellular signaling cascade is initiated within the T cell, leading to T-cell activation and differentiation.
TCR signaling alone is not sufficient to activate a naïve T cell. For activation to occur, the APC must also provide costimulation to the T cell by expressing CD80 or CD86, molecules that ligate the T-cell surface protein CD28.
The combination of signals that arises when the TCR recognizes its specific peptide and CD28 is ligated transforms a naïve T cell into an activated T cell.
T Helper (Th) Cells
Th cells are not phagocytic, cannot kill infected cells, and are incapable of the production and secretion of antibodies; instead, they coordinate the immune response by activating other immune cells.
Th cells drive the activities of other immune cells that act directly to fight infection (mac