Question
Draw and describe the structure of an antibody (immunoglobulin). Label the heavy chains, light chains, variable regions, constant regions, and antigen-binding site. What determines the specificity of an antibody?
(NCERT Class 12 — common NEET and CBSE question)
Solution — Step by Step
An antibody (immunoglobulin, Ig) has a Y-shaped structure made of 4 polypeptide chains:
- 2 heavy (H) chains — longer, ~440 amino acids each
- 2 light (L) chains — shorter, ~220 amino acids each
The chains are held together by disulphide bonds (S-S bridges). The two heavy chains are linked to each other, and each light chain is linked to one heavy chain.
Each chain has two functional regions:
- Variable region (V) — at the N-terminal (tips of the Y). The amino acid sequence here differs from antibody to antibody. This is where antigen binding occurs.
- Constant region (C) — at the C-terminal (stem of the Y). The sequence is the same within an antibody class (IgG, IgA, etc.). Determines the biological function (complement activation, binding to immune cells).
The variable regions of one H chain and one L chain together form the antigen-binding site (paratope). Since the antibody has 2 arms, it has 2 antigen-binding sites — it’s bivalent.
- Fab (Fragment antigen-binding) — the two arms of the Y. Each arm contains one complete light chain + the variable and first constant domain of the heavy chain.
- Fc (Fragment crystallisable) — the stem of the Y. Contains only the constant regions of the heavy chains. This part interacts with immune cells and complement proteins.
The hinge region between Fab and Fc gives the antibody flexibility to bind antigens at different angles.
The variable regions (specifically the hypervariable regions or complementarity-determining regions, CDRs) determine specificity. Each antibody has a unique CDR sequence that creates a unique 3D shape — this shape fits only one specific antigen (like a lock and key).
The human body can produce millions of different antibodies because of V(D)J recombination — a genetic shuffling mechanism that creates diverse variable regions from a limited number of gene segments.
Why This Works
The Y-shape is functionally elegant. The two arms bind the antigen (recognition function) while the stem communicates with the rest of the immune system (effector function). Having two binding sites means one antibody can cross-link two antigen molecules, forming large antigen-antibody complexes that are easier for phagocytes to engulf.
The separation of variable and constant regions allows the immune system to generate vast diversity (through variable region changes) while maintaining functional consistency (through constant regions that always interact with the same immune cell receptors).
Alternative Method — Quick Antibody Classes Table
For NEET, memorise the five classes of immunoglobulins:
| Class | Where found | Special feature |
|---|---|---|
| IgG | Blood, crosses placenta | Most abundant, provides passive immunity to fetus |
| IgA | Saliva, tears, breast milk | Mucosal immunity, first line in secretions |
| IgM | Blood (first responder) | Pentameric (5 units), largest Ig |
| IgE | Very low in blood | Allergic reactions, binds mast cells |
| IgD | B-cell surface | Functions as B-cell receptor |
NEET often asks: “Which Ig crosses the placenta?” (IgG) or “Which Ig is involved in allergy?” (IgE).
Common Mistake
Students often say “antibodies kill pathogens directly.” Antibodies don’t kill anything — they mark pathogens for destruction (opsonisation), block their entry (neutralisation), or activate complement proteins that do the actual killing. The antibody is the flag, not the weapon. Also, don’t confuse antigen-binding site (on the antibody, variable region) with antigenic determinant/epitope (on the antigen itself).