Figure 1. MHC-dependent antigen presentation.

CD8⁺ (left) and CD4⁺ (right) T cell receptors are activated via antigens presented by MHC I and MHC II, respectively (first signal). Together with the costimulatory signal through CD28 engagement (second signal) and cytokines (third signal), this machinery can activate different T cells into different functional subsets (bottom rows of figure). For more details on naive CD4⁺ and CD8⁺ T cell differentiation and effector functions, see reviews (6, 141). AhR, aryl hydrocarbon receptor; BCL-6, B cell lymphoma 6; Eomes, eomesodermin; Foxp3, forkhead box P3; GATA3, GATA-binding protein 3; IRF4, interferon-regulatory factor 4; ROR, retinoic acid receptor–related orphan receptor; Tc, cytotoxic T cells; TCR, T cell receptor; Tfh, T follicular helper.