IL-23 and IL-12 are heterodimeric cytokines which share the p40 subunit. Unlike IL-12, IL-23 has no effect on naive T cells but selectively activates memory T cells to secrete IFN-γ and IL-17. IL-23 is more potent than IL-12 in inducing long-lasting T-cell responses to viral antigens and can induce tumor-directed cytotoxic T-cell responses.