B Lymphocytes and Antibody Structure

• B lymphocytes originate and mature in the bone marrow.
• Mature naive B cell development: pluripotent hemopoietic stem cell → common lymphoid progenitor → pro-B cell → mature naive B cell.
• Antibodies, also known as immunoglobulins, are soluble proteins produced and secreted by B lymphocytes.
• Antibodies prevent infection by binding to infectious microorganisms.

Antibody Functions

• Inhibiting attachment to host tissue
• Activating complement
• Enhancing phagocytosis
• Inducing granulation of mast cells, eosinophils, and neutrophils

Antibody Regions and Structure

• Fab region: binds antigen
• Fc region: mediates antibody function (does not bind antigen)
• Antibodies are composed of 2 identical heavy chains (Mr 50K) and 2 identical light chains (Mr 25K), joined by di-sulphide bonds.
• The antigen-binding site is formed by adjacent N-terminal VL and VH domains within the Fab region.
• The hinge region is flexible, enhancing binding to multiple sites on the same antigen for increased affinity.
• The heavy chain's C-terminal region mediates functions like complement activation and enhanced phagocytosis.

Antigens and Epitopes

• Antigen: any molecule that specifically binds to an antibody.
• Epitope: a specific site on an antigen that binds to the antibody.
• Individual antigens can bind to different antibodies due to having several different epitopes.
• An immune complex is formed when an antibody binds to an antigen.

Antibody Binding

• The antibody V region (VH and VL domains) binds to antigen via loops between beta-strands called Complementarity Determining Regions (CDRs).

Antibody Effector Functions

• All mediated by the Fc region
• Activation of complement: C1q is activated, leading to lysis or phagocytosis via complement receptors (CR1, CR3).
• Phagocytosis of immune complexes: occurs via Fc receptors on phagocytic cells.
• Transport across epithelium to mucosal surfaces: blocks pathogen entry.
• Transfer of maternal antibodies to foetus: provides protection through the placenta.

Antibody Classes (Isotypes)

• IgM:
  • Structure: 5 antibodies with a J chain
  • Serum Level: 1.5mg/ml
  • Function: complement activation
• IgD:
  • Structure: 1 antibody
  • Serum Level: 0.04mg/ml
  • Function: signals B cells to be activated
• IgG:
  • Structure: 1 antibody
  • Serum Level: 13-14mg/ml
  • Function: complement activation, Fc receptor binding for phagocytosis, and transplacental transport
• IgA:
  • Structure: 2 antibodies with J chain
  • Serum Level: 2mg/ml
  • Function: transport across mucosal epithelium to prevent pathogen entry
• IgE:
  • Structure: 1 antibody
  • Serum Level: very low
  • Function: degranulation of immune cells to kill pathogens

Key Antibody Concentrations

• IgG: highest concentration (13-14mg/ml)
• IgM: second highest concentration (1.5mg/ml)

Complement Activation Details

• Receptors involved: C9 (pore formation for lysis) and C3b (phagocytosis of pathogen)
• Antibodies involved: IgM, IgG1, IgG2, IgG3

Mechanisms of Phagocytosis

• Fc gamma receptor binding leads to phagocytosis via a Th response.
• Antibodies involved: IgG1, IgG3

IgE and Degranulation

• IgE antibodies bind to high-affinity Fc-Epsilon receptors on mast cells and eosinophils.
• Antigen binding to the IgE/Fc receptor complex triggers degranulation (release of histamine, serotonin, proteases, cytokines, leukotrienes)
• Degranulation activates and attracts lymphocytes, eosinophils, neutrophils, and macrophages.

IgA and Mucosal Immunity

• IgA is largely present on mucosal surfaces, preventing pathogen entry.

Antibody Diversity

• Diversity is enhanced by random recombination of VDJ (heavy chain) and VJ (light chain) segments.
• Addition of nucleotides at VD, DJ, and VJ junctions (mutagenesis), forming the exon encoding the antibody V region, also contributes.

B Cell Assembly

• B cells in the bone marrow assemble cell surface forms of IgM monomer (sIgM) and IgD (sIgD) with the same antigen-binding site.
• sIgM and sIgD have different classes of heavy chains, making them specific and different.

B Cell Outcomes in Bone Marrow

• B cells that recognize self-antigens either undergo apoptosis or DNA rearrangement of surface Ig.
• B cells that weakly bind or do not recognize self-antigens migrate to the spleen and lymph nodes.
• IgD and IgM are the first antibodies produced in an antibody response.

B Lymphocyte Activation

• Antigens on macrophages or follicular dendritic cells (FDCs) are recognized by B cells via Fc receptors (immune complexes) and complement receptors (C3b, C4b).
• Antigens bind sIgM or sIgD to deliver signal 1.
• The antigen is internalized, associated with peptides and MHC II.
• Th cells recognize MHC II and send signal 2 to the B cell (CD154 binding to CD40).
• Th cells secrete cytokines that induce B cell proliferation, differentiation, and antibody production (soluble IgM and IgD).

B Cell Proliferation Outcomes

• Class switching: B cells produce different antibody classes while retaining specificity for antigen binding.
• Plasma cells: short-lived (2-3 days)
• B memory cells: long-lived and remain in circulation

Class Switching Determinants

• Determined by Th cell cytokines:
  • Th1 response (intracellular pathogens): IL-4 activates IgG1, and IL-21 activates IgG3, facilitating complement activation and Fc receptor binding.
  • Th2 response (multicellular parasites): IL-4 activates IgE, mediating degranulation of mast cells and eosinophils.
  • IgA is activated by IL-21 and transported to mucosal surfaces to block pathogen entry.

Cytokine Interactions

• IL-4 blocks IL-21, inhibiting IgA activation.

Somatic Hypermutation

• Occurs simultaneously with class switching
• Initiated by Activation Induced Cytidine Deaminase (AID), which converts cytosine to uracil.

Somatic Hypermutation Outcomes

• Diversity: mutated forms of the antigen receptor with varying affinities to the antigen
• Competition: B cells with higher-affinity antigen receptors survive and proliferate by binding to the Fc receptor of FDC.
• Apoptosis: of B cells with lower-affinity antigen receptors
• Evolution: Memory B cells produce antibodies with higher affinity.

Secondary Response

• Stronger than the primary response due to affinity maturation:
  • Class switching: ensures the most appropriate antibody is released.
  • Somatic hypermutation: competition results in high-affinity plasma and memory cells.