Mast cell activation syndrome (MCAS)

Mast cell activation syndrome is a condition in which mast cells release inflammatory mediators inappropriately, causing recurrent allergy-like episodes across several body systems.

Mast cells are immune cells that store histamine and other chemicals and normally release them in response to true threats. In this syndrome they discharge those mediators too readily, producing flushing, hives, itching, abdominal cramping, diarrhea, low blood pressure, wheezing or lightheadedness, often in repeated flares triggered by heat, foods, stress or medications. The condition is distinct from mastocytosis, in which mast cells are abnormally numerous.

Diagnosis is challenging and follows consensus criteria: symptoms involving two or more organ systems, laboratory evidence of mediator release such as a rise in serum tryptase during an episode, and improvement with medications that block those mediators. Allergists and immunologists typically lead the workup. Cannabinoid receptors are present on mast cells and have been studied in laboratory settings, but clinical research in this syndrome is very limited.