Humoral Immune Responses Summary

Humoral Immune Responses

Phases and Types of Humoral Immune Responses

Mediated by antibodies to neutralize and eliminate extracellular microbes and toxins.
Primary defense against microbes with polysaccharide and lipid capsules because B cells can produce antibodies for these, while T cells only respond to protein antigens.
Naive B lymphocytes express membrane-bound IgM and IgD as antigen receptors.
Activation leads to clonal expansion and differentiation into plasma cells, which secrete antibodies (effector cells).
Antibodies secreted have the same specificity as the surface receptors on the naive B cells that initiated the response.
One activated B cell may generate up to 4000 plasma cells, producing up to $10^{12}$ antibody molecules per day.
Heavy-chain isotype switching: B cells produce antibodies of different heavy-chain isotypes (classes) to combat different microbes.
Affinity maturation: Repeated exposure to protein antigens leads to antibodies with increasing affinity for the antigen.
Antibody responses are classified as T-dependent or T-independent based on the requirement for T cell help.

T-Dependent vs. T-Independent Antigens

T-dependent antigens: Protein antigens require helper T lymphocytes for B cell activation, isotype switching, and affinity maturation.
T-independent antigens: Polysaccharides, lipids, and nonprotein antigens stimulate antibody production without T cell help, resulting in limited isotype switching and affinity maturation.

B Cell Subsets

Follicular B cells: Reside in lymphoid organ follicles, produce T-dependent, class-switched, high-affinity antibodies to protein antigens, and generate long-lived plasma cells.
Marginal-zone B cells: Located in the splenic white pulp, respond to blood-borne polysaccharide antigens.
B-1 cells: Respond to nonprotein antigens in mucosal tissues and peritoneum.
Marginal-zone B cells and B-1 cells express antigen receptors of limited diversity and predominantly make IgM responses.

Primary vs. Secondary Antibody Responses

Primary response: Occurs after the first encounter with an antigen, resulting in smaller amounts of antibody production.
Secondary response: Occurs after repeated immunization, leading to larger amounts of antibodies, increased isotype switching, and affinity maturation (mainly in response to protein antigens).

Stimulation of B Lymphocytes by Antigen

Initiated when antigen-specific B lymphocytes recognize antigens in the spleen, lymph nodes, and mucosal lymphoid tissues.
Antigens are transported to and concentrated in B cell-rich follicles and marginal zones of peripheral lymphoid organs.
B lymphocytes recognize native (unprocessed) antigens directly via membrane-bound Ig receptors.
Antigen recognition triggers signaling pathways that initiate B cell activation.
B cell activation requires additional signals produced during innate immune reactions to microbes.

Antigen-Induced Signaling in B Cells

Antigen-induced clustering of membrane Ig receptors triggers biochemical signals transduced by receptor-associated signaling molecules.
Ig receptor-mediated signal transduction requires cross-linking of two or more receptor molecules.
Cross-linking occurs when multiple antigen molecules or repeating epitopes bind to adjacent membrane Ig molecules.
Polysaccharides, lipids, and nonprotein antigens can effectively cross-link Ig receptors due to multiple identical epitopes.

BCR Complex and Signal Transduction

Membrane IgM and IgD have short cytoplasmic tails and associate with Igα and Igβ to form the BCR complex.
Cytoplasmic domains of Igα and Igβ contain ITAMs (immunoreceptor tyrosine-based activation motifs).
Clustering of antigen receptors leads to tyrosine phosphorylation in ITAMs of Igα and Igβ by kinases.
Phosphotyrosines recruit Syk tyrosine kinase, which phosphorylates adaptor proteins and downstream signaling molecules.
Receptor-induced signaling activates transcription factors to express genes involved in B cell proliferation and differentiation.

Role of Complement Proteins in B Cell Activation

B lymphocytes express CR2 (CD21), a receptor for the complement protein fragment C3d.
Complement activation by microbes leads to coating with C3d.
B cells recognize antigen via Ig receptors and C3d via CR2 simultaneously, enhancing B cell activation.
Microbial products also directly influence B cell activation via Toll-like receptors (TLRs).

Functional Consequences of B Cell Activation by Antigen

Initiates proliferation and differentiation, preparing B cells to interact with helper T lymphocytes (if the antigen is a protein).
Activated B lymphocytes enter the cell cycle and may synthesize and secrete IgM.
Multivalent antigens (e.g., polysaccharides) strongly stimulate B cell proliferation and differentiation.
Protein antigens induce changes enhancing B cell interaction with helper T lymphocytes.
Activated B cells endocytose protein antigens, degrade them, and display peptides for recognition by helper T cells.
B cells reduce expression of chemokine receptors for lymphoid follicles and increase expression of receptors for T cell zones, migrating towards helper T cells.

Function of Helper T Lymphocytes in Humoral Immune Responses to Protein Antigens

B lymphocytes and helper T lymphocytes specific for the same antigen must interact in lymphoid organs.

T-B cell interaction steps:

CD4+ helper T cells and B cells are independently activated by a protein antigen and migrate toward each other.
Initial interaction occurs outside the follicles.
B cells process and present antigen to T cells, and helper T cells express CD40L and secrete cytokines, initiating B cell proliferation and differentiation in extrafollicular foci.
Activated B cells migrate back into the follicle, forming a germinal center, undergo somatic mutation, isotype switching, and affinity maturation.

Activation and Migration of Helper T Cells

Activated by dendritic cells and migrate towards the B cell zone.
T cell activation requires antigen recognition and costimulation.
CD4+ T cells differentiate into effector cells producing cytokines.
T cells reduce CCR7 expression and increase CXCR5 expression, while B cells decrease CXCR5 and increase CCR7 expression, facilitating migration towards each other.

Presentation of Antigens by B Lymphocytes to Helper T Cells

B lymphocytes bind protein antigens, endocytose and process them, and display class II MHC-associated peptides for recognition by CD4+ T cells.
B cells are efficient APCs for antigens they specifically recognize.
B cells recognize native epitopes, while T cells recognize peptides derived from the same antigen.
Hapten-carrier conjugates: Haptens are small chemicals recognized by B cells but require a carrier protein for strong antibody responses. Conjugate vaccines against microbial polysaccharides exploit this.

Mechanisms of Helper T Cell–Mediated Activation of B Lymphocytes

Helper T lymphocytes express CD40L and secrete cytokines, activating antigen-specific B cells.
CD40L on T cells binds to CD40 on B cells, stimulating proliferation and antibody secretion.
Cytokines bind to cytokine receptors on B cells, stimulating proliferation and Ig production.
Helper T cell signals stimulate heavy-chain isotype switching and affinity maturation.

Extrafollicular and Germinal Center Reactions

Initial T-B interaction results in low levels of antibodies.
Plasma cells are short-lived, and few memory B cells are generated.
Follicular helper T (Tfh) cells migrate into follicles, dependent on ICOS costimulation.
Tfh cells secrete cytokines such as IFN-γ, IL-4, or IL-17.
Activated B cells migrate back into the lymphoid follicle, dividing rapidly in response to signals from Tfh cells, forming a germinal center.
Germinal center B cells undergo isotype switching and somatic mutation of Ig genes.
The highest affinity B cells differentiate into memory B cells and long-lived plasma cells.

Heavy-Chain Isotype (Class) Switching

Helper T cells stimulate B lymphocytes to produce antibodies of different heavy-chain isotypes.
Different isotypes mediate different effector functions.
For example, IgG1 and IgG3 opsonize microbes for phagocytosis, while IgE targets helminths for eosinophil-mediated killing.
Induced by CD40L-mediated signals and cytokines.
X-linked hyper-IgM syndrome: Caused by mutations in the CD40L gene, leading to defective heavy-chain class switching.

Molecular basis of heavy-chain isotype switching:

IgM-producing B cells contain a rearranged VDJ gene adjacent to the Cμ constant region.
CD40 and cytokine receptor signals stimulate transcription through constant regions downstream of Cμ.
Switch regions (conserved nucleotide sequences) exist 5′ of each constant region (except Cδ).
Activation-induced deaminase (AID) converts cytosines to uracil, leading to DNA breaks and switch recombination.
Cytokines influence which heavy-chain isotype is produced.
Interferon-γ (IFN-γ): Stimulates production of opsonizing antibodies.
Interleukin-4 (IL-4): Stimulates switching to IgE.
TGF-β promotes switching to IgA in mucosal lymphoid tissues.

Affinity Maturation

Affinity of antibodies increases with prolonged exposure to protein antigens due to point mutations in V regions.
Occurs in germinal centers.
Somatic hypermutation of Ig genes in dividing B cells is followed by the selection of high-affinity B cells by antigen.
AID plays a critical role in somatic mutation, with an estimated Ig gene mutation rate of one in $10^3$ base pairs per cell per division.
Germinal center B cells undergo apoptosis unless rescued by antigen recognition and T cell help.
Follicular dendritic cells (FDCs) display antigen-antibody complexes, allowing B cells to bind antigen and be rescued.
High-affinity B cells internalize antigen, process it, and present peptides to germinal center Tfh cells.
Antibody-secreting cells (plasmablasts) migrate to the bone marrow, mature into plasma cells, and produce high-affinity antibodies for years.
Memory B cells are produced ready to respond rapidly if the antigen is reintroduced

Antibody Responses to T-Independent Antigens

Polysaccharides, lipids, and nonprotein antigens elicit antibody responses without helper T cells.
Nonprotein antigens cannot bind to MHC molecules, so they are not seen by T cells.
Multivalent arrays of epitopes may cross-link many antigen receptors, activating B cells without T cell help.
Marginal-zone B cells in the spleen and B-1 cells in mucosal tissues make T-independent responses.
Antibody responses to T-independent antigens show less heavy-chain class switching and affinity maturation compared to T-dependent.

Regulation of Humoral Immune Responses: Antibody Feedback

Activated B cells die by programmed cell death.
Antibody feedback: IgG antibodies bind to antigen, forming immune complexes.
B cells bind the antigen part via Ig receptors and the Fc tail via FcγRIIB, an inhibitory Fc receptor.
FcγRIIB delivers inhibitory signals, terminating B cell responses.