T cell receptor mediated signals in T cell activation determine the specificity, while costimulatory/accessory signals are responsible for functional results of the immune response. Given the large number of accessory receptors in the T cell membrane, there exists a potent instrument to fine-tune immune reactions. Accessory receptors and their signals represent important targets for local immune intervention, including the possibility to induce specific tolerance in vivo. This will, however, only be achieved once the molecular architecture of receptor complexes and their associations with intracellular signaling cascades are known. Here, we describe evidence for the identification of two intracellular messengers of accessory receptor-dependent signals in human T lymphocytes. Based on their functions one can begin to understand the biological significance of a critical accessory receptor-induced pathway (initially proposed as an 'alternative' pathway of T cell activation) which contributes to human T lymphocyte growth and differentiation.