Complex sentence: Commensals promote the development of B cells that produce Immunoglobulin A (IgA), which can neutralize pathogens and exotoxins and can also promote the development of TH17 and FOXP3 + regulatory T cells.
Split into simpler sentences: Commensals promote the development of B cells that produce a protective antibody, Immunoglobulin A (IgA). This can neutralize pathogens and exotoxins, and promote the development of TH17 and FOXP3 + regulatory T cells.