A recent study found that histamine increased the expression of oncostatin M (OSM), a proinflammatory mediator involved in the pathogenesis of psoriasis and AD, in M1 macrophages via the activation of H1R, H2R and H4R, and in turn, OSM stimulated STAT3 phosphorylation in human keratinocytes [28]. Here, HRH4 is linked to psoriasis.