EQUIPPING THE ARMY - The Role of Antibodies in Host Defense and Medicine

The B cell repertoire is vast.
B cells must ‘listen’ and respond to cues for activation.
B cells need to adapt their B cell receptors (BCRs) to address specific threats.

Definition of B cell repertoire: Total diversity of B cell antigen receptors derived from various specificities within the B cell population.
Clonal selection involves individual B cells responding to specific antigens leading to clonal expansion.
Clonal expansion is critical for effective immune response.

B cells require help for a successful immune response beyond mere activation.
B cell antigen receptor (BCR) plays a crucial role in the effector functions of B cells.

Classification of antibodies, also referred to as immunoglobulins, into different isotypes (IgM, IgD, IgG, IgE, IgA) that dictate various functions and roles in immune response.

Definition: Identical antibodies generated from a single clone of specific B cells producing one type of antibody.
McAb development: typically produced for therapeutic use, e.g., specific targeting of cancer markers.

Definition: A mixture of antibodies produced by different B cell clones responding to different epitopes of the same antigen.
Importance of polyclonal responses in natural infections due to the complexity of pathogens.

Clonal selection hypothesis (Frank McFarlane Burnet, 1957) outlines how individual B cells are activated by specific antigens and undergo clonal expansion.
Once activated, each clone secretes antibodies specific to the antigen encountered.

B cells produce antibodies (Ab) that can:
- Block entry of pathogens into cells (neutralization).
- Tag pathogens for destruction (opsonization).
The differentiation of B cells into Plasma B cells and Memory B cells defines their functionality:
  - Plasma B cells:
    - Reside in bone marrow, spleen, lymph nodes, and secrete antibodies at a high rate.
    - Persist for the host's lifetime.
  - Memory B cells:
    - Recirculate through blood and secondary lymphoid tissues.
    - Require T cell help for activation and differentiation as needed.

Neutralization: Prevents microbes or toxins from entering or damaging host cells.
Activation of Complement: Antibodies can activate complement pathways leading to pathogen clearance.
Opsonization: Enhances phagocytosis by marking pathogens.
ADCC (Antibody-Dependent Cell-Mediated Cytotoxicity): Trigger target cell killing by NK cells.
Degranulation of Mast Cells: Mediates allergic responses or expulsion of pathogens.

Diversity in antibody repertoire produced via
- V(D)J recombination in B cells leading to a substantial diversification of antibody specificities.
The recombination of gene segments (V, D, J) leads to the formation of functional heavy and light chains in antibodies.
Heavy chain loci located on Chromosome 14, with the light chains separated across chromosomes 2 (kappa) and 22 (lambda).

Antibodies consist of two identical heavy chains and two identical light chains, creating two identical antigen-binding sites.
Distinctions in regions:
- Variable (V) region: responsible for antigen binding.
- Constant (C) region (also called Fc region): dictates the class and function of the antibody.

Maternal antibodies provide neonatal immunity:
- IgG: crosses the placenta, providing passive immunity in utero.
- IgA: found in breast milk, securing mucosal immunity in infants.

Each antibody class has distinct pathways and mechanisms:
  - IgM: First antibody produced, effective in complement activation.
  - IgA: Main immunoglobulin in mucosal surfaces; anti-inflammatory and protective.
  - IgE: Mediator of allergic responses and defense against large parasites.
  - IgG: Dominant in secondary responses & effective in opsonization and ADCC.

Dominant tool for therapeutic interventions due to superior specificity and controlled production.
Case Study:** Rituximab, a mAb targeting CD20, uses ADCC to treat B cell lymphoma.
Challenges overcoming non-human antibodies leading to immune responses: Development of humanized monoclonal antibodies retains specificities while reducing immunogenicity.

Continuous advancements in the use of mAbs for various therapeutic interventions.
The importance of enhancing specificity and reducing immunogenicity of mAbs for patient safety and effectiveness.