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Activation of Helper T Cells Helper T cells only recognize antigen in the grasp of Class II MHC markers. An antigen-presenting cell--such as a macrophage or a dendritic cell--breaks down the antigen it devours, then it places small pieces (peptides) on its surface along with a Class II MHC marker. By exhibiting its catch in this way, antigen-presenting cells enable specific receptors on helper T cells to bind the antigen and confirm (via CD4 protein) that an invasion has occurred. After binding, a resting helper T cell quickly becomes an activated helper T. It assumes command of the immune response, giving orders to increase the number of specific antibody-producing plasma cells and the cytotoxic killer cells needed to quell the attack.
Activation of Cytotoxic T Cells 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.
Source: National Cancer Institute, USA.
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