Germline transcription of Ighg1, Ighg3, Ighg2b, and Ighg2a/c was greatly enhanced in CD5- B-1b cells from A/J compared to C57BL/6J mice on day 5 of secondary infection (Fig 6F), suggesting heightened isotype class switching occurred in mice in the resistant background. Here, IGHG1 is linked to infection.