Class I MHC proteins play a role in the cellular immune response that is similar to the role played by class II MHC proteins in the humoral immune response. In a virus-infected or mutated cell, foreign or abnormal proteins or peptide fragments combine with class I MHC molecules. The resulting complex is displayed on the cell surface and presented to T_C cells. When a T_C cell recognizes and binds to this antigen–MHC I complex, it is activated to proliferate (Figure 41.13B).

In the effector phase of the cellular immune response, T_C cells recognize and bind to cells bearing the same antigen–MHC I complex. These bound T_C cells produce a substance called perforin, which ultimately results in apoptosis of the bound target cell. In addition, the T_C cells can bind to a specific receptor (called Fas) on the target cell that initiates apoptosis in that cell. These two mechanisms work in concert to eliminate the antigen-containing host cell. Because T_C cells recognize MHC proteins complexed with nonself antigens, they help rid the body of its own virus-infected or cancer cells.