Which immune cell primarily recognizes peptides presented by MHC Class I molecules?

• A. B cells
• B. CD4+ T cells
• C. NK cells
• D. CD8+ T cells

Answer: (D)

Peptide presentation by MHC Class I is recognized by CD8+ cytotoxic T cells. MHC Class I molecules display short peptide fragments that come from proteins made inside the cell, including viral proteins during infection. These peptide–MHC I complexes appear on almost all nucleated cells. The T cell receptor on CD8+ T cells specifically binds to this complex, and the CD8 co-receptor helps stabilize the interaction to activate the cytotoxic response, leading to killing of the presenting cell if the peptide is foreign. This is distinct from CD4+ T cells, which recognize peptides presented by MHC Class II on professional antigen-presenting cells, and from B cells, which recognize native antigens. NK cells are part of the innate system and respond to changes in MHC I expression, not to peptide–MHC I complexes via a T cell receptor.