RAG1 and infection: IgM was also detectable in the lungs of wild-type and Rag1−/− mice that received B cells, but levels were significantly higher in the wild type C57Bl/6 mice (Fig. 3f).Lung IgM (Fig. S1B) and serum GXM-IgM (Fig. S1C) were also detectable 14 d post-infection in mice that received peritoneal B cells.