CD19 and influenza: To investigate the contribution of CD5- B cells to local IgM secretion, we FACS-sorted CD19+IgM+IgDloCD43+ CD5+ and CD5- B cells on days 3, 5, and seven after influenza infection from C57BL/6 mice (Figure 1D), which were then cultured for 2 days to analyze spontaneous IgM secretion by ELISA.