Q.An antibody molecule is drawn as a Y-shaped structure built from four polypeptide chains held together by inter-chain disulfide (–S–S–) bridges. Two longer chains run from the tips of the two arms down through the central stem of the Y; two shorter chains lie along the outer edge of each arm. At the tip of each of the two arms there is an identical cleft, and the amino (N) terminals sit at the arm tips while the carboxyl (C) terminals sit at the base of the stem. In this drawing, pointer A indicates the cleft at the tip of one arm, pointer B indicates one of the two shorter polypeptide chains, and pointer C indicates one of the two longer chains that form the stem. Name the parts labelled A, B and C.

Each arm tip carries a variable-region cleft that binds antigen (A). The two shorter polypeptides are the light chains (B) and the two longer polypeptides running through …

Concept

An antibody is a globular protein secreted by plasma cells (activated B-lymphocytes) as part of the humoral immune response. Every antibody monomer is built from four polypeptide chains: two identical long (heavy) chains and two identical short (light) chains. Because it contains two heavy (H) and two light (L) chains, an antibody is represented as H₂L₂. The chains are linked to one another by covalent inter-chain disulfide (–S–S–) bonds, giving the characteristic Y shape.

Reading the structure

• Each chain has a variable region (at the arm tips, near the N-terminals) and a constant region (toward the stem, ending at the C-terminals).
• The variable regions of one heavy and one light chain together form a cleft at the tip of each arm. There are two such clefts per molecule, so one antibody can bind two identical antigenic determinants (it is bivalent).

Identifying the labelled parts

1. A points to the cleft at the tip of an arm — this is the antigen binding site (the paratope), formed by the variable regions of the heavy and light chains; it recognises and binds a specific antigen (epitope). …

Derive the labels from the H₂L₂ formula and molecular geometry rather than reading them off the picture alone: 4 chains total, in exactly two distinct lengths — count 2 long + 2 short. The 2 LONGER chains must run through the stem, since the stem is the shared, thicker part of the Y — that gives label C (heavy chain). The 2 SHORTER chains sit only along the outer arm edges — that gives label B (light chain). Sin …