IFN-γ is known to play a role in antibody production and isotype switching in response to an infection (19, –, 22), suggesting that the diminished antibody-mediated protection observed during secondary infection in CD4-depleted IFN-γ−/− mice could be due to a different, less protective antibody response in the absence of IFN-γ. The gene discussed is IFNG; the disease is infection.