CD40LG and infection: Additionally, cytokines such as GM-CSF and IL-3, produced by IgM+ and IgG+ B1b B cell plasmablasts, play an important role in the immune response.362 Early in the infection, these cytokines are primarily produced by IgM+ B1b B cells, with a later shift to IgG+ plasmablasts, suggesting an isotype switch and highlighting the functional plasticity and phenotypic heterogeneity of innate B1 B cell subsets.362