Fig. 1.

Costimulation between T cells and antigen-presenting cells. Schematic overview of signal 1, 2 and 3 of T-cell activation. During signal 2, costimulatory molecules on T cells and antigen-presenting cells interact to activate or inhibit T cells after alloantigen recognition. Two important groups of costimulatory molecules are presented: the immunoglobulin superfamily and the TNF/TNFR superfamily. The costimulatory molecules discussed in this review are green and the costimulatory molecules that are not discussed are yellow. Several biologicals are developed that interfere with the costimulatory molecules on T cells and antigen-presenting cells. CTLA 4 cytotoxic T lymphocyte protein 4, HLA human leucocyte antigen, ICOS inducible T-cell costimulator, PD programmed death, SLAM signaling lymphocytic activation molecule, TCR T-cell receptor, TIM T cell/transmembrane, immunoglobulin, and mucin, TNF tumor necrosis factor, TNFR tumor necrosis factor receptor