Skin Inflamed. Lesional skin samples from patients with psoriasis were immunohistochemically stained for interleukin-9 (IL-9) (red). Schlapbach et al. report that, in humans, a subset of T helper cells that produce IL-9, TH9 cells, are mainly tropic for the skin. These cells may function normally to protect against extracellular pathogens, but aberrant activation of these cells may contribute to inflammatory diseases of the skin.