CD40LG and infection: Thus, while the number of B cells remained stable in the blood of fish injected with phosphate-buffered saline (PBS) and ranged between 1.3 and 3 × 103 IgM+ lymphocytes/μl of blood, the infection induced an increase from 21 dpi onwards, peaking at 13 × 104 IgM+ B cells/μl of blood (approx. 60-fold increase), at 56 dpi (Fig. 2).