Ans. is 'b' i.e., 2 Activation of CD4+ T Cells: Naive CD4+ T 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 acti- vation. In some situations the APCs (dendritic cells and macrophages) produce IL-12, which induces differentiation of CD4+ T cells to the TH1 subset. IFN-y produced by these effector cells promotes fuher 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.