Clonal mast cell proliferation occurs in the bone marrow, skin, and other organs.- Often associated with mutations in the KIT receptor tyrosine kinase.- Cells characterized by prominent expression of mast cell tryptase.

Excessive histamine release mediates syncope, flushing, hypotension, pruritus, urticaria. In addition, histamine induces gastric acid secretion, which can lead to gastric ulceration. The excess acid also inactivates pancreatic intestinal enzymes, causing diarrhea.