Color enhanced scanning electron micrograph (SEM) of mast cells. Specialized immune response cells, mast cells are found in the tissues of the dermis (skin). They play a central role in inflammatory and immediate allergic reactions by releasing allergen inflammatory mediators such as histamine, proteases, chemotactic factors, cytokines, and metabolites of arachidonic acid. These act on the vasculature, smooth muscle, connective tissue, mucous glands, and inflammatory cells. An itchy red rash on the skin is a common response of mast cells to a skin allergen.