In the 1950s, Witebsky established the criteria for determining the aetiology of human diseases which were thought to be autoimmune. Witebsky's postulates were modelled on those of Koch and required the presence of autoantibody or a cell-mediated immune response to a self-antigen, which having been identified, could induce a similar disease in an experimental host mediated by the same immunological mechanism. Since these criteria were established, our knowledge of immunology has increased by leaps and bounds. However, they remain a good guide for identifying autoimmune diseases.
Clinically, autoimmune diseases have been divided into systemic or ''non-organ-specific" and ''organ-specific'' diseases. However, this categorisation is not absolute. Many autoimmune diseases have both organ-specific and non-organ-specific complications. In addition, more than one type of disease may occur in the same individual.
The classical example of a non-organ-specific autoimmune disease is systemic lupus erythematosus (SLE) in which the autoimmune response is directed to a number of different tissue antigens. Such non-organ-specific diseases may develop as a result of an abnormal immune response to a single antigen, which is expressed in many tissues.
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