An important step in activation of naive CD4 +T cells and initiation of cell-mediated immune response is played by Interleukin-

Activation of CD4 +T cells: Naive CD4 +T cells cells recognize peptides displayed by dendritic cells and secrete IL-2, which functions as an autocrine growth factor to stimulate proliferation of the antigen-responsive T cells. The subsequent differentiation of antigen-stimulated T cells to TH1 or TH17 cells is driven by the cytokines produced by APCs at the time of T-cell activation. In some situations, the APCs (dendritic cells and macrophages) produce 1L-12, which induces the differentiation of CD4 +T cells to the TH1 subset. IFN-γ produced by these effector cells promotes further TH1 development, thus amplifying the reaction. If the APCs produce inflammatory cytokines such as IL-1, IL-6, and a close relative of IL-12 called IL-23, these stimulate differentiation of T cells to the TH17 subset.