Mastocytosis is actually a spectrum of rare disorders, all of which are characterized by – not surprisingly – an increase in mast cells. Most patients have disease that is localized to the skin, but about 10% of patients have systemic involvement. There is a localized, cutaneous form of mastocytosis called urticaria pigmentosum that happens mostly in children and accounts for over half of all cases of mastocytosis.

Clinically, the skin lesions of mastocytosis vary in appearance. In urticaria pigmentosum, the lesions are small, round, red-brown plaques and papules. Other cases of mastocytosis show solitary pink-tan nodules that may be itchy or show blister formation. The itchiness is due to the release of mast cell granules (which contain histamine and other vasoactive substances).

In systemic mastocytosis, patients have skin lesions similar to those of urticaria pigmentosum – but there is also mast cell infiltration of the bone marrow, lymph nodes, spleen and liver. Patients often suffer itchiness and flushing triggered by certain foods, temperature changes, alcohol and certain drugs (like aspirin).

Histologically, mast cell infiltration may be subtle (with ocasional spindle-shaped mast cells around vessels in the superficial dermis) to dramatic (with tightly-packed mast cells in the upper to mid-dermis – check out the photo above). Mast cells may be difficult to distinguish in regular H&E sections; you may need a metachromatic stain (toluidine blue or Giemsa) to really visualize the granules.