Fig. 3: A Cytotoxic T-lymphocyte Recognizing a Virus-Infected Cell

Endogenous antigens are those being produced within cells of the body. Examples include proteins from replicating viruses, proteins from intracellular bacteria, and tumor antigens. The body marks infected cells and tumor cells for destruction by placing peptide epitopes from these endogenous antigens on their surface by way of MHC-I molecules. Cytotoxic T-lymphocytes (CTLs) are then able to recognize peptide/MHC-I complexes by means of their T-cell receptors (TCRs) and CD8 molecules and kill the cells to which they bind.

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