IL21 and bacterial infectious disease: Previous studies have shown that lipopolysaccharide (LPS), interferon-γ (IFN-γ) and transforming factor-α (TNF-α) can induce monocytes to produce the M1 phenotype, while interleukin-4 (IL-4), interleukin-10 (IL-10), interleukin-13 (IL-13) transforming factor-β (TNF-β), bacterial infection, colony-stimulating factor 1 (CSF-1) and interleukin-21 (IL-21) are used to produce the M2 phenotype.