Antibodies are Y-shaped proteins crucial for immune defense. They have a unique structure with heavy and light chains, forming antigen-binding (Fab) and crystallizable (Fc) regions. This design allows them to recognize specific threats and trigger various immune responses.

Different antibody isotypes (IgG, IgM, IgA, IgE, IgD) have specialized functions in the body. They work through mechanisms like opsonization, complement activation, and neutralization to protect against pathogens. Antibody affinity and avidity play key roles in determining their effectiveness.

Antibody Structure and Function

Components of antibody structure

• Immunoglobulin (Ig) structure forms Y-shaped molecule with two identical heavy chains and two identical light chains
• Constant and variable regions comprise Fab (antigen-binding) region containing variable domains of heavy and light chains with hypervariable regions (complementarity-determining regions, CDRs) and Fc (crystallizable) region containing constant domains of heavy chains
• Disulfide bonds stabilize structure through interchain and intrachain connections
• Hinge region provides flexibility for antigen binding
• Glycosylation sites attach N-linked oligosaccharides on Fc region influencing effector functions

Functions of antibody isotypes

• IgG neutralizes toxins and viruses, promotes opsonization, and mediates antibody-dependent cell-mediated cytotoxicity (ADCC) as most abundant isotype in serum, crosses placenta
• IgM initiates primary immune response with pentameric structure enabling efficient complement activation
• IgA protects mucosal surfaces through dimeric structure in secretions, neutralizes pathogens (respiratory tract, gut)
• IgE triggers allergic reactions, defends against parasites, binds to mast cells and basophils
• IgD functions as B cell receptor component, participates in B cell activation and maturation

Antibody-mediated effector mechanisms

• Opsonization enhances phagocytosis by coating pathogens with antibodies, facilitating Fc receptor recognition
• Complement activation initiates classical pathway through C1q binding to antibody-antigen complexes, leading to membrane attack complex formation
• Neutralization prevents cell entry or receptor interaction by binding to toxins or viral particles
• ADCC triggers NK cell recognition of antibody-coated targets, resulting in release of cytotoxic granules
• Antibody-dependent cellular phagocytosis (ADCP) promotes phagocyte recognition of antibody-coated pathogens
• Mast cell and basophil degranulation releases inflammatory mediators through IgE-mediated activation

Antibody affinity in immune responses

• Affinity measures strength of single antigen-antibody binding site interaction, influenced by complementarity of binding site and epitope, non-covalent interactions (hydrogen bonds, van der Waals forces)
• Affinity maturation improves antibody effectiveness through somatic hypermutation in B cells, selecting high-affinity clones
• Higher affinity leads to stronger antigen binding, improving pathogen neutralization and clearance
• Avidity represents overall strength of antibody-antigen interaction, influenced by antibody valency and epitope density
• Kinetics of antibody-antigen interactions described by association rate ($k_{on}$), dissociation rate ($k_{off}$), and equilibrium dissociation constant ($K_D = k_{off} / k_{on}$)
• High-affinity monoclonal antibodies developed for diagnostics and therapeutics (cancer treatment, autoimmune disorders)