CD40LG and infection: In addition, indirect evidence of significant up-regulation of antigen-presenting-capacity related genes, including MHC II, as well as CD83 and CD80/86, in phagocytic IgM+ B cells have been described in a number of teleost species in response to various pathogenic bacteria or viruses, which indicates that teleost IgM+ cells act as APCs during the course of a pathogenic infection (56, 90, 91).