Mast Cell Activation Syndrome (MCAS)
A condition where mast cells release chemical mediators (histamine and others) inappropriately, producing flushing, GI symptoms, fatigue and anaphylactoid reactions. Strongly comorbid with POTS and hEDS.
Reviewed 2026-05-15
What mast-cell activation looks like
Mast cells release histamine and other mediators as part of the normal allergic response. In MCAS, that release happens inappropriately and chronically, producing flushing, itching, GI upset, dizziness, fatigue, and (less commonly) full anaphylactoid reactions to triggers that wouldn't bother most people.
Why it appears on a POTS site
MCAS frequently coexists with POTS and hEDS, and the symptom overlap (flushing, tachycardia, GI distress) means people often need to be evaluated for all three. Diagnosis of MCAS is contested in the literature — the consensus criteria (Castells et al., Valent et al.) require objective evidence of mediator release on top of the clinical picture.