The major subsets of CD4+ T cells that differentiate from naïve circulating cells. Naïve T cells differentiate into at least four functional subsets following stimulation by antigen presented by dendritic cells, which are specialized for driving the activation of T cells and are thought to help direct their differentiation by differential secretion of cytokines determining the different subsets. Three subsets - TH1, TH2 and TH17-activate other immune cells with distinct roles in immunity, including B cells, which secrete antibody, natural killer (NK) cells, which are important in defense against viruses, and inflammatory cells, such as neutrophils and macrophages (which also have non-inflammatory functions). The fourth subset shown here comprises regulatory T cells (Tregs), which suppress the activation of the other subsets, partly by acting on dendritic cells. Modified from Figure 5-22 in DeFranco AL, Locksley RM, Robertson M: Immunity: The Immune Response in Infectious and Inflammatory Disease. London: New Science Press; 2007.