Figure 2.

CD4⁺ T cell activation. Three signals are necessary to fully activate CD4⁺ T cells. The first signal is mediated by the interaction between TCRs and peptide/MHC-II complexes at the APC surface. The second signal is mediated by costimulatory molecules, such as CD80 on APCs, which interacts with CD28 on CD4⁺ T cells. The third signal is delivered by soluble factors including cytokines, which are notably secreted by the APCs.