Killer T cells only recognize antigen in the grasp of Class I MHC markers. Here a resting cytotoxic T cell recognizes virus fragments, which are displayed by a macrophage in combination with a Class I MHC marker. A receptor on a circulating, resting cytotoxic T cell (and CD8 protein) recognizes the antigen-protein complex and binds to it. The binding process and an activated helper T cell activate the cytotoxic T cell. Because the surfaces of other infected cells bear the same virus fragments in combination with Class I MHC markers, the activated cytotoxic T cells can quickly recognize, attack, and destroy the diseased cell.
![Activation of T Cells: Cytotoxic](https://webarchive.library.unt.edu/eot2008/20080923085222im_/http://www.nci.nih.gov/images/Documents/02853881-1148-4541-b2b6-e8ecd3e50c6f/cancer23.jpg)
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