CD40LG and infection: In both lymph nodes the population of IgM+ cells among all FM+ GC B cells decreased from about 40% after 1st infection to less than 10% in the memory phase and after 2nd infection, while IgG1+ cells increased from 30% after 1st infection to 50% in the memory phase and about 70% after 2nd infection (Figure 2A; S1B, C).