Initiation and Progression of the Pathogenic Process
Autoimmune reactions are common phenomena during the development of a cognate immune response. The characteristic of these events, however, is that they stop immediatelyoncetheexogenoustriggeringantigeniseliminated. An autoimmune disease occurs, however, when the immune reaction has grown sufficiently to sustain itself even in the absence of exogenous antigen. As several workers have at least partially suggested in the past, and as Matzinger articulated as a theory in 1994 and demonstrated later(Ridgeetal.,1996),theimmunesystemdoesnotreally seemtodiscriminatebetweenselfandnonself,butbetween whatisorisnotdangerous.Theimmunesystemrecognizes the antigen, irrespective of whether it comes from an exogenous or an endogenous protein, but depending on whether it is properly presented (i.e. at a sufficiently high concentration and in the presence of costimulatory signals). Danger signals (e.g. tissue stress, death or destruction) allow, by virtue of activation of professional antigen-presenting cells, the efficient presentation of self antigens that were previously ignored by the immune system. Therefore, autoimmunity may be seen less as a defect in the immune response and more as the way in which the
Autoimmune Disease: Pathogenesis
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self antigen is presented. The theory of ‘danger’ (Matzinger, 1994) combines most of the hypotheses described above on how the breakdown of tolerance is obtained and maintained, and explains many of the remaining doubts. How the danger signal is maintained still remains to be clarified. Indeed, in this model, it must be hypothesized that professional antigen-presenting cells continue to presentself-orcross-reactingexogenousantigensefficiently. See also: Immunological danger signals