Among CD4+ effector T-cells, T-helper 1 (Th1) cells are critical for host defense against intracellular pathogens and secrete interferon-γ (IFN-γ), whereas T-helper 2 (Th2) cells control parasitic infections and secrete interleukin 4 (IL-4), IL-5 and IL-13, important for mucosal barrier function, as well as for the induction of B-cell proliferation. The gene discussed is IL4; the disease is parasitic infectious disease.