As mentioned above, T cells help control the immune response and kill infected cells. Infected cells are recognized because they chop up foreign proteins from the invader and display the bits on their surface. These bits, which are antigens, are held aloft by surface proteins, called MHC (major histocompatibility complex) proteins. The MHC-antigen complex is recognized by the T cell receptor, in cooperation with one or more other T cell surface molecules.
When a T cell discovers a cell whose MHC proteins contain foreign antigens, it marks the cell for destruction. The T cell receptor interacts with antigens in much the same way as an antibody does, although the size of the antigen it recognizes is smaller. T cell receptors come in as many diverse forms as antibodies do, and, while the details differ, their diversity is generated in much the same way, with random recombination of gene segments.
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