Figure 1.

Activation of naïve T cells requires (a) T cell receptor (TCR)–peptide–MHC interaction (signal 1) and (b) co-stimulation (signal 2) for full activation. This can be provided by so-called professional antigen-presenting cells (APCs; i.e. dendritic cells, macrophages and B cells). In the absence of co-stimulation the T cells will become anergic.