[Biological switches: the 'all-or-none' principle in T-cell activation].

T helper lymphocytes (Th cells) play a central role in cellular defence against pathogens as well as in cellular self tolerance. The activation of Th cells is a crucial process determining the course of a protective immune response. Dysregulated activation processes can lead to pathologic immune reactions and may induce autoimmune diseases. Thus, for example, chronic activation of Th cells can trigger autoimmune diseases such as rheumatoid arthritis. One fundamental feature of antigen-specific T-cell activation is the synthesis of cytokines, e.g. Interleukin- (IL-)2. Here we present results of our working group indicating for the first time that, at the level of the individual cell, IL-2 is expressed in a binary fashion, i.e. according to an all-or-none principle. The identification and characterization of such intracellular switches may provide new options to manipulate the fate of T cells and develop novel therapeutic strategies.