Immuno Week 2 Learning Objectives

What is V(D)J recombination?

V(D)J recombination is the process by which developing B and T cells rearrange Variable (V), Diversity (D), and Joining (J) gene segments to generate highly diverse antigen receptors.

What gene segments are present in an immunoglobulin heavy chain locus?

The immunoglobulin heavy chain locus contains Variable (V), Diversity (D), and Joining (J) gene segments.

What gene segments are present in immunoglobulin light chains?

Immunoglobulin kappa and lambda light chains contain Variable (V) and Joining (J) gene segments but no Diversity (D) segments.

What gene segments are used to generate TCR diversity?

TCR beta and delta chains use V, D, and J segments, whereas TCR alpha and gamma chains use V and J segments.

What is the 12/23 rule?

The 12/23 rule ensures that V(D)J recombination occurs only between gene segments flanked by recombination signal sequences (RSS) separated by either 12 or 23 base pairs, preventing improper gene-segment joining.

What are RAG-1 and RAG-2?

RAG-1 and RAG-2 are proteins that initiate V(D)J recombination by creating double-stranded DNA breaks at recombination signal sequences.

What is junctional diversity?

Junctional diversity results from random addition or deletion of nucleotides at gene-segment joining sites during V(D)J recombination. It greatly increases the diversity of antigen receptors.

Why is junctional diversity important?

Junctional diversity allows lymphocytes to generate receptors capable of recognizing a vast range of novel antigens, including pathogens the host has never encountered.

What are KRECs?

KRECs are Kappa-deleting Recombination Excision Circles, circular DNA fragments produced during B-cell maturation that can be detected as evidence of successful B-cell development.

What are TRECs?

TRECs are T-cell Receptor Excision Circles, circular DNA fragments produced during T-cell maturation that can be detected as evidence of successful T-cell development.

What are KRECs and TRECs used for clinically?

KRECs and TRECs are used in neonatal screening to assess successful lymphocyte production and detect immunodeficiency. They can also help assess immune reconstitution after bone marrow transplantation or gene therapy.

What happens when V(D)J recombination machinery is defective?

Defects in V(D)J recombination, such as RAG mutations or defects in DNA repair enzymes, can cause severe combined immunodeficiency (SCID) with profound impairment of functional lymphocyte development.

What is SCID?

Severe Combined Immunodeficiency (SCID) is a severe immunodeficiency characterized by profound impairment or absence of functional B and T cells, resulting in vulnerability to severe opportunistic infections.

What happens during the pro-B-cell stage?

During the pro-B-cell stage, immunoglobulin heavy-chain rearrangement begins. Pro-B cells express CD10 and CD19.

What characterizes the pre-B-cell stage?

The pre-B-cell stage is characterized by expression of the pre-B-cell receptor (pre-BCR), consisting of a successfully rearranged heavy chain paired with a surrogate light chain.

What characterizes an immature B cell?

An immature B cell has successfully rearranged its light chain and expresses surface IgM as its B-cell receptor. It also expresses CD20.

What characterizes a mature B cell?

A mature B cell expresses both IgM and IgD, gains CD21, and migrates to peripheral lymphoid organs.

What is CD10?

CD10 is a surface marker expressed during early B-cell development and is also known as the common ALL antigen.

What is CD19?

CD19 is a B-cell surface marker expressed during B-cell development and on mature B cells.

What is CD20?

CD20 is expressed on immature and mature B cells and is a marker of the B-cell lineage.

What is CD21?

CD21, also called complement receptor 2 (CR2), is gained during mature B-cell development and is expressed on mature B cells.

What is receptor editing?

Receptor editing is a central tolerance mechanism in which a self-reactive immature B cell undergoes additional light-chain rearrangement to change the specificity of its B-cell receptor.

What happens to strongly self-reactive immature B cells?

Strongly self-reactive immature B cells can undergo clonal deletion through apoptosis.

What is B-cell anergy?

Anergy is a state of functional unresponsiveness that can occur when B cells recognize self-antigens with relatively low affinity.

What factors influence B-cell tolerance mechanisms?

B-cell tolerance mechanisms are influenced by the physical structure of the self-antigen, such as whether it is multivalent or soluble, and by the developmental timing of antigen exposure.

What is the role of CD40L in B-cell activation?

CD40 ligand (CD40L) on activated CD4+ T cells binds CD40 on B cells, promoting B-cell proliferation and germinal-center formation.

What is the hapten-carrier effect?

The hapten-carrier effect describes how a small non-immunogenic molecule, called a hapten, can elicit an antibody response when chemically linked to a larger protein carrier that provides T-cell epitopes.

Why is a carrier protein required for a hapten to generate a strong antibody response?

The carrier protein provides peptide epitopes that can be presented on MHC class II to helper T cells, allowing T-cell help for the B cell recognizing the hapten.

What is AID?

Activation-Induced Cytidine Deaminase (AID) is an enzyme required for somatic hypermutation and class-switch recombination in activated germinal-center B cells.

Where does AID act during class-switch recombination?

AID acts at GC-rich switch regions of immunoglobulin genes and initiates DNA changes required for class-switch recombination.

What is somatic hypermutation?

Somatic hypermutation introduces point mutations into immunoglobulin variable regions of activated B cells, allowing selection of B cells with higher-affinity antibodies.

What is affinity maturation?

Affinity maturation is the process by which B cells producing progressively higher-affinity antibodies are selected during the germinal-center response following somatic hypermutation.

What is class-switch recombination?

Class-switch recombination changes the immunoglobulin constant region, such as switching from IgM to IgG, while preserving antigen specificity.

What is the role of UNG in class-switch recombination?

UNG removes uracil generated by AID-mediated cytidine deamination during class-switch recombination.

What is the role of MSH2/6 in class-switch recombination?

MSH2/6 recognizes the mismatches generated during AID-mediated DNA modification during class-switch recombination.

What is the role of APE1/APE2 in class-switch recombination?

APE1/APE2 are endonucleases that create DNA breaks during class-switch recombination.

What DNA repair mechanism joins the broken DNA during class-switch recombination?

Non-Homologous End Joining (NHEJ) joins the DNA segments during class-switch recombination.

What causes Bruton agammaglobulinemia?

Bruton agammaglobulinemia is caused by an X-linked defect in BTK, resulting in failure of B-cell maturation and absence of all immunoglobulin classes.

What is the immunologic defect in Bruton agammaglobulinemia?

BTK deficiency prevents normal B-cell maturation, resulting in markedly reduced or absent mature B cells and immunoglobulins.

What causes Hyper-IgM syndromes?

Hyper-IgM syndromes result from defects in class-switch recombination, including defects in CD40L, AID, or UNG.

What is the immunoglobulin pattern in Hyper-IgM syndrome?

Hyper-IgM syndrome is characterized by elevated IgM with markedly decreased or absent IgG, IgA, and IgE because class switching is impaired.

How can abnormal SHM or CSR contribute to malignancy?

Dysregulated somatic hypermutation or class-switch recombination can produce inappropriate immunoglobulin gene changes and is associated with B-cell lymphomas.

Where does positive selection of T cells occur?

Positive selection occurs in the thymic cortex.

What is the purpose of positive selection?

Positive selection ensures that developing T cells can recognize self-MHC with appropriate affinity, establishing MHC restriction.

What happens during positive selection?

Developing T cells that recognize self-MHC with moderate affinity receive survival signals and continue development.

Where does negative selection of T cells occur?

Negative selection occurs primarily in the thymic medulla.

What is the purpose of negative selection?

Negative selection eliminates developing T cells that bind self-antigen-MHC complexes too strongly, helping prevent autoimmunity.

What happens to T cells that strongly recognize self during negative selection?

They are deleted through apoptosis.

What is MHC restriction?

MHC restriction means that T cells recognize antigen only when it is presented in association with the body's own MHC molecules.

What markers characterize T cells?

T cells express the CD3 complex, which is associated with the T-cell receptor and is a major marker of the T-cell lineage.

What are the major functions of Th1 cells?

Th1 cells produce IFN-gamma and activate macrophages, promoting cell-mediated immunity against intracellular pathogens.

What cytokine is characteristic of Th1 cells?

IFN-gamma is the characteristic cytokine produced by Th1 cells.

What are the major functions of Th2 cells?

Th2 cells produce IL-4, facilitate B-cell responses, and promote defense against helminths.

What cytokine is characteristic of Th2 cells?

IL-4 is a characteristic cytokine produced by Th2 cells.

What are the major functions of Th17 cells?

Th17 cells promote inflammation and participate in defense against fungi.

What are CTLs?

Cytotoxic T lymphocytes (CTLs) are CD8+ T cells that directly kill infected or tumor cells.

What are the characteristic markers of Tregs?

Regulatory T cells express CD4, CD25, and FoxP3.

What is the function of Tregs?

Tregs suppress immune responses and help maintain immune homeostasis and self-tolerance.

What is the difference between alpha-beta and gamma-delta T cells?

Alpha-beta and gamma-delta T cells differ in the TCR chains they express. Gamma-delta T cells are often found at mucosal surfaces.

Where do NK T cells develop?

NK T cells develop in the thymus.

What makes NK T cells unique?

NK T cells express markers associated with NK cells and recognize lipid antigens presented by CD1d rather than conventional peptide-MHC complexes.

What cytokine promotes Th1 differentiation?

IL-12 promotes Th1 differentiation.

What cytokine promotes Th2 differentiation?

IL-4 promotes Th2 differentiation.

What is required for generation of CD4+ helper T cells and CTLs?

Generation of CD4+ helper T cells and CTLs requires antigen presentation by dendritic cells, costimulatory signals such as CD80/86 binding CD28, and appropriate polarizing cytokines.

What is the primary immune response?

The primary response occurs during the first exposure to an antigen and is characterized by a lag phase and initial production of relatively low-affinity IgM.

What is the secondary immune response?

The secondary response occurs after re-exposure to an antigen and is faster and stronger, with predominantly high-affinity, class-switched antibodies such as IgG.

What is original antigenic sin?

Original antigenic sin describes the immune system's tendency to preferentially use memory cells generated during an initial exposure when encountering a mutated version of the pathogen.

Why is original antigenic sin important for vaccines?

Original antigenic sin can complicate vaccine design because pre-existing immune memory may influence responses to updated viral strains.

How do CTLs kill target cells?

CTLs release perforin and granzymes. Perforin creates membrane pores, while granzymes activate apoptotic pathways including the caspase cascade.

What is the role of perforin?

Perforin creates pores in the target-cell membrane, facilitating entry of cytotoxic molecules.

What is the role of granzymes?

Granzymes are proteases released by CTLs that enter target cells and activate apoptotic pathways, including caspases.

What happens to excess T cells after antigen clearance?

After antigen clearance, excess effector T cells undergo apoptosis to limit inflammation and restore immune homeostasis.

What are clinical red flags for primary immunodeficiency?

Recurrent severe infections, opportunistic infections, failure to thrive, and chronic diarrhea can suggest primary immunodeficiency.

What infections suggest a B-cell or complement defect?

Recurrent pyogenic infections suggest a B-cell or complement defect.

What infections can suggest a T-cell or phagocyte defect?

Opportunistic infections such as Pneumocystis jirovecii or Cryptosporidium can suggest a T-cell or phagocyte defect.

How does stress affect immune function?

Elevated cortisol during stress can suppress lymphocyte function.

How does malnutrition affect immunity?

Malnutrition deprives the immune system of the nutrients and building blocks required for rapid immune-cell proliferation and function.

What is immunosenescence?

Immunosenescence is the age-associated decline in immune function, including a shrinking T-cell repertoire.

What does flow cytometry measure?

Flow cytometry can measure cellular characteristics such as Forward Scatter (FSC), which reflects cell size, and Side Scatter (SSC), which reflects cellular granularity.

What does Forward Scatter indicate in flow cytometry?

Forward Scatter (FSC) primarily reflects cell size.

What does Side Scatter indicate in flow cytometry?

Side Scatter (SSC) primarily reflects cellular granularity or internal complexity.

What are clinical uses of flow cytometry?

Flow cytometry is used to assess CD4+ counts in HIV, diagnose paroxysmal nocturnal hemoglobinuria, detect fetal RBCs in maternal blood, and evaluate hematologic abnormalities.

What is Type I hypersensitivity?

Type I hypersensitivity is an immediate IgE-mediated reaction. Examples include anaphylaxis and asthma.

What is Type II hypersensitivity?

Type II hypersensitivity is antibody-mediated tissue injury involving IgG or IgM antibodies binding to cell-surface or extracellular-matrix antigens. An example is autoimmune hemolytic anemia.

What is Type III hypersensitivity?

Type III hypersensitivity results from deposition of immune complexes in tissues, activating complement and inflammation. Examples include SLE and rheumatic fever.

What is Type IV hypersensitivity?

Type IV hypersensitivity is a delayed, T-cell-mediated immune response. Examples include contact dermatitis and multiple sclerosis.

What causes Type I hypersensitivity?

Type I hypersensitivity occurs when IgE binds Fc receptors on mast cells. Re-exposure causes IgE cross-linking, mast-cell degranulation, and mediator release.

What is the role of mast cells in Type I hypersensitivity?

Mast cells bind IgE through Fc receptors. Antigen-mediated cross-linking of IgE triggers mast-cell degranulation and release of inflammatory mediators such as histamine.

What does histamine do in Type I hypersensitivity?

Histamine promotes immediate inflammation and increases vascular permeability.

How does Type II hypersensitivity cause tissue injury?

Type II injury can occur through complement-mediated lysis or antibody-dependent cellular cytotoxicity (ADCC).

What is ADCC?

Antibody-Dependent Cellular Cytotoxicity is a mechanism in which immune cells such as NK cells or macrophages recognize antibody-coated target cells and contribute to their destruction.

How does Type III hypersensitivity cause tissue injury?

Immune complexes deposit in tissues such as vessel walls, activate complement, and recruit neutrophils. Neutrophils undergo frustrated phagocytosis and release damaging inflammatory mediators.

What is immune surveillance?

Immune surveillance is the concept that the immune system identifies and eliminates nascent malignant cells.

How do NK cells recognize tumor cells?

NK cells can recognize cells that have downregulated MHC molecules, which is a common tumor immune-evasion strategy.

How do CTLs recognize tumor cells?

CD8+ CTLs recognize tumor-associated or tumor-specific antigens presented on MHC class I molecules.

How do tumors evade immune detection?

Tumors can downregulate MHC expression, secrete immunosuppressive cytokines such as TGF-beta, and recruit Tregs to suppress local immune responses.

What is the role of TGF-beta in tumor immune evasion?

TGF-beta is an immunosuppressive cytokine that tumors can secrete to dampen anti-tumor immune responses.

How do Tregs contribute to tumor immune evasion?

Tumors can recruit Tregs to the tumor microenvironment, where Tregs suppress immune responses against tumor cells.

What is hyperacute transplant rejection?

Hyperacute rejection occurs within minutes and is caused by pre-existing recipient antibodies that recognize graft antigens and activate complement.

What is acute transplant rejection?

Acute rejection occurs over days to weeks and is primarily T-cell mediated.