IL-33, also known as “alarmin” and belonging to the IL-1 family, is also reported to be increased in psoriatic lesions.159 IL-33 is mainly produced by non-hematopoietic cells such as keratinocytes and fibroblast cells, but is also expressed by hematopoietic cells, especially mast cells (MCs) and DCs.160 In psoriasis, MCs can be activated by IL-1 and then produce TNF and IL-33, the latter in turn activating MCs in psoriatic inflammation.161 The secreted IL-33 also binds to its receptor, ST2 receptor, to promote the proliferation of keratinocytes, which aggravates the psoriasis. This evidence concerns the gene TNF and psoriasis.