Which statement correctly describes MHC class I function?

Which statement correctly describes MHC class I function?

• A. They present exogenous peptides to CD4+ T cells.
• B. They are expressed exclusively on B cells.
• C. They present endogenous peptides to CD8+ T cells.
• D. They present lipid antigens to T cells.

Answer: (C)

MHC class I molecules display peptides derived from proteins made inside the cell (endogenous), and these peptide–MHC complexes are recognized by CD8+ cytotoxic T cells. Proteins in the cytosol are degraded by the proteasome into short peptides, which are transported into the endoplasmic reticulum by TAP, loaded onto MHC I with β2-microglobulin, then transported to the cell surface. This presentation allows CD8+ T cells to detect infected or abnormal cells and initiate killing. In contrast, presenting exogenous peptides to CD4+ T cells is the role of MHC class II, MHC I is not restricted to B cells but is found on nearly all nucleated cells, and lipid antigens are presented by CD1 molecules rather than MHC class I.

MHC class I molecules display peptides derived from proteins made inside the cell (endogenous), and these peptide–MHC complexes are recognized by CD8+ cytotoxic T cells. Proteins in the cytosol are degraded by the proteasome into short peptides, which are transported into the endoplasmic reticulum by TAP, loaded onto MHC I with β2-microglobulin, then transported to the cell surface. This presentation allows CD8+ T cells to detect infected or abnormal cells and initiate killing. In contrast, presenting exogenous peptides to CD4+ T cells is the role of MHC class II, MHC I is not restricted to B cells but is found on nearly all nucleated cells, and lipid antigens are presented by CD1 molecules rather than MHC class I.