Question
Describe the structure of an antibody (immunoglobulin) molecule. Explain the roles of the variable and constant regions, and name the five classes of immunoglobulins.
(NEET 2021, similar pattern)
Solution — Step by Step
An antibody (immunoglobulin) is a Y-shaped glycoprotein produced by plasma cells (differentiated B lymphocytes). Each antibody molecule consists of:
- 4 polypeptide chains: 2 identical heavy (H) chains and 2 identical light (L) chains, linked by disulfide bonds.
- The molecule has a characteristic Y-shape: the two arms of the Y are the Fab regions (Fragment antigen binding), and the stem is the Fc region (Fragment crystallizable).
Each chain has two distinct regions:
Variable region (V region):
- Located at the tips of the Y-arms (N-terminal end of both H and L chains).
- This is where antigen binding occurs — the antigen-binding site.
- The amino acid sequence varies enormously between different antibodies, giving each antibody its unique specificity. The most variable parts are called hypervariable regions or complementarity-determining regions (CDRs).
- Each antibody has 2 identical antigen-binding sites (one on each arm) — it is bivalent.
Constant region (C region):
- Makes up the rest of the molecule (the stem and the lower portions of the arms).
- The amino acid sequence is the same for all antibodies of the same class.
- Determines the biological function — which immune cells can bind it, whether it crosses the placenta, activates complement, etc.
- The Fc region (constant region of heavy chains) binds to Fc receptors on immune cells like macrophages and mast cells.
Between the Fab and Fc regions is the hinge region — a flexible segment that allows the two arms to move independently. This flexibility helps the antibody bind to two antigen molecules simultaneously, even when they are different distances apart on a pathogen surface.
| Class | Location | Key Feature |
|---|---|---|
| IgG | Blood, tissue fluid | Most abundant (~80%). Crosses placenta (provides passive immunity to fetus). Activates complement. |
| IgA | Secretions (saliva, tears, breast milk, mucus) | Secretory antibody. Protects mucosal surfaces. Dimeric form (two Y-units joined). |
| IgM | Blood (mainly) | First antibody produced in immune response. Pentameric (5 Y-units joined). Best at activating complement. |
| IgE | Bound to mast cells, basophils | Involved in allergic reactions and defence against parasitic worms. Triggers histamine release. |
| IgD | Surface of B cells | Acts as a receptor on B cells. Role in B cell activation. Present in very low levels in blood. |
Why This Works
The antibody structure is a masterpiece of molecular design. The variable region provides virtually unlimited diversity (the immune system can produce antibodies against millions of different antigens), while the constant region provides standardised effector functions. This modular design means the immune system can target any pathogen while using the same downstream mechanisms (complement activation, phagocyte recruitment) for destruction.
The bivalent nature allows antibodies to cross-link antigens, forming large antigen-antibody complexes (agglutination) that are easily engulfed by phagocytes.
NEET commonly tests: “Which immunoglobulin crosses the placenta?” Answer: IgG. “Which is the first antibody produced?” Answer: IgM. “Which is involved in allergic reactions?” Answer: IgE. “Which is found in breast milk?” Answer: IgA. Learn these associations as flashcards.
Common Mistake
Students often confuse the Fab and Fc regions. The Fab region (arms of Y) binds the antigen — it has the variable region. The Fc region (stem of Y) determines the class and biological function — it interacts with immune cells and complement. A simple mnemonic: Fab = binds the Foreign antigen, Fc = Classification (determines antibody class).
Another error: writing that each antibody has one antigen-binding site. Each antibody molecule has two identical antigen-binding sites (bivalent) — one on each arm of the Y. IgM, being pentameric, has 10 binding sites (5 x 2), which is why it is so effective at agglutination.