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Cell-mediated immunity

While B cells and the antibodies they produce play a vital role in the immune response, they do not act alone. As noted above, T lymphocytes, or T cells, constitute another part of the specific defense system. Lymphocytes normally do not react with “self” cells, that is, those that belong in the body. This is due to the presence of a group of proteins present on all “self” cells called the major histocompatibility complex (MHC), which lymphocytes are able to recognize. The MHC proteins also play a role in the functioning of T cells. A T cell cannot interact with an antigen unless that antigen is “presented” by one of the body’s own cells that has become infected by the pathogen. For example, if a virus infects a cell, the viral antigens will not be freely circulating in the blood or lymph to be detected by B cells or antibodies. However, the infected cell displays new antigens on its surface. Like B cells, each T cell displays a surface antigen receptor, similar to an antibody, that can bind and recognize only one type of antigen. When a T cell encounters a cell presenting an antigen along with the MHC complex, the T cell becomes stimulated. Thus, T cells are responsible for cell-mediated immunity.


There are several classes of T cells, each of which performs a different function in the cell-mediated response. T cells, like B cells, rapidly divide and differentiate after being stimulated, creating cells of the following types:
  • Effector or cytotoxic T cells act by directly attacking and destroying the infected cell, and also release chemicals called lymphokines which attract and stimulate macrophages.
  • Memory T cells have the same function as do memory B cells.
  • Helper T cells release chemicals called interleukins after being stimulated. These chemicals act to increase the activities of both cytotoxic T cells and B cells, ensuring a speedy and potent response to infection. (It is these helper T cells that are infected by the HIV virus in AIDS, causing a general suppression of the immune response.)
  • Suppressor T cells seem to play a role in shutting off the immune response after the pathogen has been eliminated, but this process is not yet completely understood.

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