Which statement correctly describes the structural features of MHC class I and II molecules?

• A. MHC-I: symmetric alpha and beta chains; MHC-II: large alpha chain with beta-2 microglobulin
• B. MHC-II: symmetric alpha and beta chains; MHC-I: large alpha chain with beta-2 microglobulin
• C. MHC-I: heavy chain only; MHC-II: heavy chain only
• D. MHC-I: single chain; MHC-II: two chains

Answer: (B)

MHC class I molecules are built from a single, large heavy chain that forms the peptide-binding groove with its α1 and α2 domains, and this heavy chain is noncovalently associated with a separate, non-membrane-anchored protein called beta-2 microglobulin, which stabilizes the complex. In contrast, MHC class II molecules are a true heterodimer made from two different polypeptides, an α chain and a β chain, each contributing to the peptide-binding groove and both spanning the membrane.

This description matches the correct choice: MHC-II consists of alpha and beta chains, while MHC-I consists of a large alpha heavy chain associated with beta-2 microglobulin. The other options misstate the chain composition or the involvement of beta-2 microglobulin.