Many immune responses can be destructive to host tissue; hence, it is vital that they be restricted to pathogens and not be raised against innocuous substances. The devastating consequences of autoimmune diseases, which result from immune responses that are inappropriate in that they are directed against self, illustrate the crucial need for self-tolerance. It is now known that the immune system has the inherent capability of responding not only to foreign but also to self antigens. During development, it 'learns' not to respond to self antigens. The two major mechanisms for establishing self-tolerance are clonal deletion, the killing of self-reactive lymphocytes, and clonal anergy, the functional inactivation of self-reactive lymphocytes involving antigen stimulation in the absence of accessory signals. These processes are focused mainly on T lymphocytes since most B cells require helper T cells to respond to antigen so that the elimination of self-reactive helper T cells also ensures the inactivation of self-reactive B cells.
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