Figure 1:

A simplistic model of the subset differentiation of T helper cells. The differentiation is initiated by contact with cognate antigen peptide–MHC complex on the antigen presenting cell. The direction of differentiation is largely determined by cytokines, the effects of which are relayed by specific STAT proteins. The STATs, together with downstream lineage-specifying transcription factors, orchestrate subset-specific gene expression programs, including production of hallmark cytokines.