Only recently recognised, mast cell activation syndrome (MCAS) is a large, prevalent collection of illnesses resulting from mast cells (MCs) which are inappropriately activated but which, in contrast to the (collectively rare) forms of mastocytosis, are not significantly proliferating.

Due to the diversity of direct and indirect, local and remote effects of the menagerie of mediators released by MCs, likely due to highly heterogeneous sets of mutations in MC regulatory elements, MCAS typically presents as chronic, persistent or recurrent, waxing/waning or slowly progressive, generally inflammatory multisystem polymorbidity.

Initial manifestations often occur in childhood but are non-specific; in fact, virtually all of the syndrome’s manifestations are non-specific, leading to decades of mysterious illness complicated by incorrect or superficial diagnoses often poorly responsive to empiric therapies.

Diagnosis is further challenged in detecting specific biomarkers of MC activation other than serum tryptase levels, which usually are elevated in systemic mastocytosis but normal in MCAS. MCAS therapy aims to inhibit mediator production/release and block/ ameliorate mediator action.

Although patient-specific optimal therapy is not presently predictable, a methodical, persistent, trial-and-error approach usually finds helpful therapy.

Lifespan with MCAS approximates normal, but quality of life can be significantly impaired without correct diagnosis and effective treatment.