Cytotoxic T Cell

Simplified illustration of a cytotoxic T cell (upper right) identifying an HIV-infected T-helper cell (lower left). When a T-helper cell has been infected by the AIDS virus, small pieces of viral molecules, called epitopes (blue), become bonded to the HLA proteins and then transported to the infected cell's surface. After recognizing an HLA/viral-epitope combination with a receptor of complementary shape, the cytotoxic T cell will kill the infected helper. HLA proteins are thus indispensable in the process by which HIV-infected cells are identified and killed. Although only one viral epitope is recognized by a given cytotoxic-T-cell receptor, an individual's immune system potentially responds to dozens of different epitopes; in addition, different epitopes will be recognized by the immune systems of different individuals.

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