Antibodies (Immunoglobulins)
Antibodies (immunoglobulins) are glycoproteins capable of binding antigens with high specificity and affinity. Humans produce five major classes: IgG, IgA, IgM, IgD, IgE, each with distinct roles and structural features.
Plasma vs Serum
- Plasma = Blood fluid with clotting factors.
- Serum = Plasma – fibrinogen (clotting factor removed).
Basic Structure of Immunoglobulins
Core structure
Each antibody consists of a basic 4-chain unit:
- 2 identical Light chains (L)
- 2 identical Heavy chains (H)
- Chains are linked by inter-chain and intra-chain disulphide bridges.
- Non-covalent interactions also help stabilize the molecule.
Functional regions
- Variable region (V region)
– Located on both H & L chains.
– Contains the antigen-binding site.
- Constant region (C region)
– Determines class (isotype) of antibody.
– Controls effector functions (complement activation, phagocyte binding, etc.).
Proteolytic fragments
- Fab fragment – Binds antigen.
- Fc fragment – Responsible for complement activation & binding to immune cells.
Affinity & Valence
Affinity
- Describes how tightly an antibody binds to its antigen.
- Higher affinity = more stable binding.
- Memory responses produce higher-affinity antibodies.
Valence
- Number of antigenic determinants an antibody can bind.
- More binding sites = stronger total binding (increased avidity).
- Example: Two linked sites on IgG neutralize viruses 100× more effectively than two unlinked sites.