CD4 and viral infectious disease: Briefly, in the presence of interleukin (IL)-12 and IFN-γ, which are typically produced in response to viral infection, effector CD4+ T cells polarize to the Th1 subset, characterized by production of IFN-γ, tumour necrosis factor (TNF)-α and IL-2, and expression of the T-box transcription factor, T-bet [31].