Recent studies have demonstrated that IgM is the predominant Ig isotype involved in the immune response to pathogens in systemic compartments (e.g. serum, spleen, and head kidney), where significant accumulations of IgM+ B cells were found after infection with parasitic and bacterial pathogens (Xu et al., 2016; Xu et al., 2013; Yu et al., 2018). The gene discussed is CD40LG; the disease is infection.