Infected CD28−/loxCre+/− mice given tamoxifen prior to secondary infection had a reduced number of CD19+B220+ B cells (Fig. 4C) due to strikingly reduced numbers of follicular B cells compared to infected wild-type mice (Fig. 4D) or infected untreated CD28−/loxCre+/− littermate control mice (Fig. S1G). This evidence concerns the gene PTPRC and infection.