CD40LG and bacterial infectious disease: Given that much higher proportions of IgT+ B cells than IgM+ B cells were observed in the trout OO (4) and that, similar to the mammalian IgA, IgT is a specialized mucosal Ig isotype against parasitic infection in the gut, gills, and skin of rainbow trout (7, 9, 35), we hypothesized that IgT may also play a key role in teleost NALT against bacterial infection.