On day 21 post-infection, compared with day 0, wild-type mice exhibited an increase in serum IgM, IgG, and lung IgM, and serum and lung IgM were each detectable in Rag1−/− mice that received B cells 14 and 21 d post-infection, albeit at much lower levels than in wild-type mice. The gene discussed is RAG1; the disease is infection.