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Autoimmunity is the term for the immune conditions characterized by a specific humoral or cell mediated response to the body's own tissues. The termination of the natural state of self tolerance may lead to immunopathological manifestations with clinical consequences, i.e. autoimmune diseases. In a very general sense, one may classify autoimmune diseases into two groups with respect to the underlying mechanism: 1. There are autoimmune diseases which develop in the presence of a normal intact regulation mechanism. 2. Another group whose development must be understood on the basis of a cellular dysfunction. In the first case, dequestered or semi-sequestered autoantigens are liberated as a consequence of exogenic influences inducing the sensitization of immunocompetent cells. The immune system then reacts with these autoantigens in the same way as with foreign substances. This kind of autoimmune disease will, however, not be dealt with here. In the second case, autoantigens are normally, i.e. in healthy individuals, accessible to the immunocompetent cells. To understand the reason for the development of an autoimmune reaction one must first clarify the mechanism of self tolerance. Then one must examine the way in which a break of this physiological state takes place. One of the major unanswered questions is the relative importance of antibody-mediated and cell-mediated immune mechanisms in the onset and further development of autoimmune diseases. Recently it has been suggested that a dysfunction at the cellular level might represent the basic cause which induces the termination of selftolerance. Most of the conceptions about the mechanism by which autoimmune diseases are triggered were gained through experiments with animals. It is, however, difficult to use these experimental results to explain human diseases; in humans many questions are still open. Undoubtedly, the mechanisms of induction and maintenance of self tolerance and also the ways in which autoimmune diseases may be induced, are not uniform. In all these cases, cells and cellular interactions as well as the corresponding cellular products are decisive. The majority of autoimmune diseases are mediated by antibodies as can be demonstrated in transfer experiments, for instance. Experimental Autoimmune Thyroiditis (EAT), rather than by sensitized cells. An example of the latter would be Experimental Autoimmune Encephalitis (EAE). In principle the following can be said of all these kinds of autoimmune diseases as well as of selftolerance: 1. Induction of autoantibodies is in principle possible. 2. Self antigens important in autoimmune diseases are T-dependent. 3. Self-reacting lymphocytes (T- and/or B-cells) are present in "normal" individuals.