S100A9 and psoriasis: Upon stimulation and the effect of the feed-forward mechanism, keratinocytes in the skin produce various kinds of cytokines (Il36, Il1b, Ill17c, Il22, and Il8), chemokines (Ccl2, 3, 5, 17, 20, 22, and Cxcl1, 2, 5, 9, 10, 11), and AMPs (S100a7, S100a8, S100a9, Defb4, Defb14, Lcn2, and Camp), which play critical roles in the pathogenesis of psoriasis.