Systemic Mastocytosis is due to excessive mast cell stimulation. It leads to anaphylactic like states - urticaria, flushing and also GI symptoms such as diarrhoea and nausea. Mast cells are Ig E mediated but can be triggered by injury, drugs and complements.

It is caused by mast cell release of histamine in the skin or connective tissue. Diagnosis can be confirmed by raised levels of urinary N-methyl imidazole, blood eosinophilia and thrombocytopenia. It classically presents with episodes of flushing, vomiting, diarrhoea and abdominal pain.