CD40LG and infection: It has been reported that fish myeloid cells and IgM+ B cells phagocytize microbes.[49, 50] In the present study, serum IgM from E. piscicida‐infected tilapia, especially after secondary infection, coated the pathogen (Figure 2B,C) and enhanced the phagocytosis by myeloid cells (Figure 2D) and IgM+ B cells (Figure 2E,F).