Both Il4−/− and Il4ra−/− mice have normal numbers of T and B cells, indicating that IL-4 and IL-13 are not required for development of these lineages, but they have diminished IgG1 production and greatly decreased IgE production, after infection with Nippostrongylus brasiliensis, whereas levels of IgM and other IgG isotypes are produced at normal levels16,17. The gene discussed is IL13; the disease is infection.