CD19 and infection: However, after 10 days of infection, while more than 70% of B cells expressed IgM, immature pro-B progenitors (CD19+/loIgM−CD43+/hi) were seen to grow in number as the infection progressed, but their differentiation into pre-B cells appeared to be delayed since no proportional increase was observed in CD19+IgM−CD43−/lo (Figure 5B, right panel).