Among the HLA-Class II molecules, which statement is correct?

The statement that the DRA chain can combine with several different DRB chains is correct. HLA-Class II molecules are composed of an alpha chain and a beta chain, designated as DRA and DRB respectively. The DRA chain is non-polymorphic, meaning it does not vary much between individuals, which allows it to pair with various beta chains (e.g., DRB1, DRB3, DRB4, and DRB5). This ability to pair with different DRB chains enhances the diversity of the HLA-Class II molecules present on the cell surface and subsequently affects the immune response.

The other statements do not accurately reflect the characteristics of HLA-Class II molecules. The polymorphism noted in HLA-Class II typically occurs in the DRB gene, particularly in exon 2, which participates directly in the formation of the peptide-binding site alongside the beta chain. While DRB3, DRB4, and DRB5 are indeed polymorphic, the most polymorphic gene is commonly considered to be DRB1. The peptide-binding site is formed by contributions from both the alpha and beta chains, meaning that it is incorrect to state that only the B chain is responsible for this structure.