Paper I
Question
Enumerate the various classes of immunoglobulins. Write briefly on the structure and properties of IgG (2+2+4)
Answer
Classes of immunoglobulins: IgG, IgA, IgM, IgD, and IgE — five distinct classes (isotypes), distinguished by their heavy chain constant region.
Structure of IgG:
- Basic monomeric four-chain structure: two identical heavy chains (γ chains) and two identical light chains (κ or λ), linked by disulfide bonds, forming a “Y”-shaped molecule.
- Each chain has a variable region (Fab, antigen-binding) and a constant region (Fc, effector function).
- The heavy chain constant region has three domains (CH1, CH2, CH3), distinguishing it from IgM/IgE (which have four).
- Molecular weight approximately 150 kDa; four subclasses exist: IgG1, IgG2, IgG3, IgG4, differing in hinge region structure and effector functions.
Properties of IgG:
- Most abundant immunoglobulin in serum (approximately 75–80% of total serum immunoglobulin).
- Predominant antibody of the secondary immune response, with high affinity due to affinity maturation.
- The only immunoglobulin class that crosses the placenta, providing passive immunity to the neonate during the first few months of life.
- Fixes complement (via the classical pathway), most efficiently by IgG1 and IgG3.
- Mediates opsonization (Fc receptor-mediated phagocytosis) and antibody-dependent cell-mediated cytotoxicity (ADCC).
- Has a relatively long serum half-life (approximately 21–23 days), longer than the other immunoglobulin classes, contributing to its role in long-term humoral immunity/memory.
- Distributed both intravascularly and extravascularly (unlike IgM, which is largely confined to the intravascular compartment).