CD40LG and infection: In a recent study with C57BL6 "bumble" mice, deficient in the transcription factor Nfkbid on which B-2 and B-1 cells depend for class switch and development, respectively, do not produce IgM antibodies and succumb to a secondary infection by T. gondii. If they are reconstituted with peritoneal B-1 cells from wildtype donors, they survive longer than non-transferred animals and produce IgM antibodies, which bind to tachyzoites and inhibit their invasion to 3T3 mouse fibroblasts (Fc receptor [FcR] negative) [114].