We also studied the relative distribution of IgT1+, IgT2+ and IgM+ B cells in the gills, gut, spleen and head kidney of healthy carp and preliminarily assessed the responses of the different B cell subsets to a systemic infection with the blood parasite T. borreli by analyzing the presence of parasite-specific antibodies in serum and B cell proliferation and distribution in spleen, one of the most affected organs by this infection. The gene discussed is CD40LG; the disease is infection.