- A Two identical heavy polypeptide chains alone, joined together without any light chains present
- B Four polypeptide chains: 2 heavy chains + 2 light chains, held by disulfide bonds, forming a Y shape
- C Six separate polypeptide chains arranged together in a closed hexagonal ring-like structure
- D A single continuous polypeptide chain folded back to form two separate antigen-binding regions
Correct answer: B. Four polypeptide chains: 2 heavy chains + 2 light chains, held by disulfide bonds, forming a Y shape
Explanation: Antibodies (immunoglobulins) have 4 polypeptide chains: 2 identical heavy chains and 2 identical light chains, connected by disulfide bonds. The variable regions at the tips of the Y form the antigen-binding sites (2 per antibody). The Fc region determines class and effector function.
Concept context
Innate and adaptive immunity, B cells, T cells, antibodies, vaccines, and immune disorders. Critical for NEET.