Which statement about MHC Class I is correct?

• A. MHC Class I molecules are on the membranes of nucleated cells and help identify self
• B. MHC Class I molecules are only on red blood cells
• C. MHC Class I molecules present extracellular peptides to CD4+ T cells
• D. MHC Class I molecules secrete antibodies

Answer: (A)

MHC Class I molecules are found on the surface of almost all nucleated cells and serve as a display case for peptides generated inside that cell. By presenting these intracellular peptides to CD8+ cytotoxic T cells, they allow the immune system to monitor cell health and detect viruses or other abnormal proteins. Cells with normal, self-peptides are kept in check, while cells presenting foreign or altered peptides can be targeted for destruction. They are not on red blood cells (which lack MHC I), they don’t present extracellular peptides to CD4+ T cells (that’s the role of MHC II), and they don’t secrete antibodies (produced by B cells). So the statement that MHC Class I molecules are on nucleated cell membranes and help identify self best captures their role.