Which class of molecules presents peptides to CD8+ T cells and is expressed on almost all nucleated cells?

• A. MHC class I
• B. MHC class II
• C. CD1 molecules
• D. TLR receptors

Answer: (A)

Antigen presentation to CD8+ T cells is done by MHC class I molecules. These are found on nearly all nucleated cells, which lets cytotoxic T cells patrol the body for cells producing abnormal or foreign peptides, such as from viral infections or transformed cells. Inside the cell, proteins are degraded into peptides and transported into the endoplasmic reticulum by TAP transporters, where they bind to the MHC class I heavy chain (with beta-2 microglobulin) and are presented on the cell surface. The CD8+ T cell receptor recognizes this peptide–MHC I complex, triggering a cytotoxic response that can kill the presenting cell. In contrast, MHC class II molecules present extracellular peptides to CD4+ T helper cells and are mainly on professional antigen-presenting cells, while CD1 molecules present lipid antigens, and TLR receptors are innate immune sensors.