Cytotoxic T-cells kill infected cells by recognizing what marker presented on the target cell?

• A. Antibiotics
• B. MHC I-presented peptides
• C. Surface lipids
• D. Complement proteins

Answer: (B)

Cytotoxic T-cells recognize infected or abnormal cells by detecting specific peptide fragments displayed on the cell surface by MHC class I molecules. Inside the cell, proteins are processed into peptides and loaded onto MHC I, which then presents them on the surface. When a CD8+ T cell’s receptor binds a peptide-MHC I complex, the T cell becomes activated and can kill the target cell by releasing perforin and granzymes or by triggering apoptosis through Fas-FasL interactions. This is how infected cells are flagged for destruction while the surrounding cells are spared.

Antibiotics aren’t markers presented by cells to T cells; they’re drugs, not peptides displayed on the surface. Surface lipids aren’t the typical markers recognized by cytotoxic T-cells, and while certain lipid antigens can be presented by specialized pathways, the classic mechanism for recognizing infected cells is peptide-MHC I presentation. Complement proteins are part of the humoral/complement cascade, not the peptide-MHC marker that T-cells inspect.