What is the role of major histocompatibility complex (MHC) molecules in the immune response?

• A. MHC molecules present antigen fragments to T-cells, enabling T-cell activation.
• B. MHC molecules produce antibodies to neutralize pathogens.
• C. MHC molecules directly kill infected cells.
• D. MHC molecules regulate fever and inflammatory responses.

Answer: (A)

Antigen presentation by MHC molecules to T cells is what kickstarts the adaptive immune response. MHC proteins display short peptide fragments on the surface of cells, coming from proteins made inside the cell or from proteins taken up from the environment. T cells survey these peptide–MHC complexes; when a T cell receptor recognizes a specific peptide bound to the right MHC molecule, the T cell becomes activated, proliferates, and differentiates into effector cells that coordinate targeted defense against the pathogen or infected cell. There are two main pathways: MHC I presents endogenous peptides to CD8+ cytotoxic T cells, while MHC II presents exogenous peptides to CD4+ helper T cells, enabling both killer and helper functions in the immune response. The other options describe roles outside of antigen presentation: antibodies are produced by B cells, not directly by MHC; killing of infected cells is performed by cytotoxic cells after they recognize peptide–MHC complexes, not by MHC itself; and fever/inflammation are driven by cytokines and innate signals rather than MHC regulation.