This area of immunology is difficult to define and remains elusive. Antigens represent the most important positive regulator of immunity; since there is simply no immune stimulation when antigens have been eliminated or are absent. Other important regulators include interferon gamma (IFNy) for TH1 responses, and IL-4 forTH2 responses. Further IL-dependent regulatory functions are in the process of being defined. The existence of specific CD8+ T suppressor cells, capable of downregulating immune responses, has been postulated and their role was assumed to be that of counteracting the inflammatory CD4+ T cell response. However, to date there has been no convincing proof of their existence. The term CD8+ T suppressor cells, which is used frequently, is therefore misleading and inaccurate. In relatively rare cases, cyto-

Kayser, Medical Microbiology © 2005 Thieme

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toxic CD8+ T cells do exercise a regulatory effect by lysing infected APCs or B cells (see also p. 106). It is unclear whether CD4+ T cells could have similar effects. Regulation via idiotypic/anti-idiotypic antibody networks (i.e., antibodies directed against the ABS of other antibodies), or anti-TCR networks, have also been postulated—but remain hypothetical. Although attractive hypothesis, for most cases such regulatory pathways have only proved disappointing theoretical concepts, and as such should no longer be employed in the explanation of immunoregulation. In isolated cases, anti-idiotypic, or anti-TCR peptide-specific feedback, mechanisms can be modeled under forced experimental conditions. However such conditions probably fail to model normal situations, therefore they cannot accurately indicate whether these feedback mechanisms have a role in regulating the immune system as a whole.

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