The yin and yang of T cell costimulation.

Focuses on T cells' two types of signals from antigen-presenting cells (APCs) for activation and subsequent differentiation to effector function. Balanced interplay between positive signals from CD28 and negative signals from CTLA-4; Reason for the CD28 and CTLA-4 receptors; Inhibitory signalling role for CTLA-4; Apoptosis in the homeostatic regulation of T cell responses.