CD4 and viral infectious disease: TH1 CD4+ T cells produce the key TH1 cytokines interleukin-2 (IL-2), interferon-γ (IFNγ) and tumor necrosis factor-α, and this response is widely implicated in bacterial and viral infections, while TH2 CD4+ T cell produce key TH2 cytokines IL-4, IL-5 and IL-13, being the key response in parasite infections and allergic reactions [1].