Which statement accurately describes MHC-I?

Which statement accurately describes MHC-I?

• A. MHC-I is expressed on red blood cells and presents to CD8+ T cells
• B. MHC-I is expressed on all nucleated cells and presents peptides to CD8+ T cells
• C. MHC-I is expressed only on professional APCs and presents to CD4+ T cells
• D. MHC-I presents peptides to CD4+ T cells

Answer: (B)

MHC class I presents endogenous peptides to CD8+ cytotoxic T cells and is expressed on all nucleated cells. This broad expression allows immune surveillance of intracellular pathogens and abnormal proteins inside nearly every cell. Peptides presented by MHC I are derived from proteins in the cytosol, processed by the proteasome, and transported into the endoplasmic reticulum by TAP, where they load onto MHC I with the help of beta-2 microglobulin and then travel to the cell surface for recognition by CD8+ T cells. Red blood cells lack a nucleus and do not express MHC class I, which is why the statement about RBCs is incorrect. In contrast, MHC class II is expressed mainly by professional antigen-presenting cells and presents to CD4+ helper T cells, not CD8+ T cells.

MHC class I presents endogenous peptides to CD8+ cytotoxic T cells and is expressed on all nucleated cells. This broad expression allows immune surveillance of intracellular pathogens and abnormal proteins inside nearly every cell. Peptides presented by MHC I are derived from proteins in the cytosol, processed by the proteasome, and transported into the endoplasmic reticulum by TAP, where they load onto MHC I with the help of beta-2 microglobulin and then travel to the cell surface for recognition by CD8+ T cells. Red blood cells lack a nucleus and do not express MHC class I, which is why the statement about RBCs is incorrect. In contrast, MHC class II is expressed mainly by professional antigen-presenting cells and presents to CD4+ helper T cells, not CD8+ T cells.