Mast Cell Activation Syndrome (MCAS) is a condition in which mast cells inappropriately and excessively release chemical mediators, resulting in a range of chronic symptoms, sometimes including anaphylaxis or near-anaphylaxis attacks. MCAS is an umbrella term that describes a set of symptoms; it is not a specific diagnosis. Multiple diagnostic schemes for MCAS have been proposed, and it has been increasingly over-diagnosed or misdiagnosed. MCAS is considered a multi-systemic condition, which means that two or more body systems are affected at the same time during a reaction. Due to the wide-ranging symptoms associated with MCAS, there can be an overlap with the symptoms experienced with a number of other conditions. During any diagnostic process, it is important to ensure that other potential diagnoses are not missed.

MCAS can present with a wide range of symptoms in multiple body systems, including cardiovascular, dermatological, gastrointestinal, neurological, and respiratory problems. Symptoms typically wax and wane over time, varying in severity and duration. Many signs and symptoms are the same as those for mastocytosis, because both conditions result in too many mediators released by mast cells.

MCAS is an inflammatory condition that affects multiple systems. It is common for someone with MCAS to have mild and controllable symptoms for a long time, but a stressful event like an infection, an operation, or an illness can cause a dramatic increase in symptom severity or frequency.

There is no permanent cure available for MCAS, and management is based on the avoidance of triggers and medication to help control symptoms. Treatments can include antileukotrienes, nonsteroidal anti-inflammatory drugs, and natural products such as curcumin or St. Johns wort extracts. Some people believe that a low FODMAP diet can help MCAS, but this is not yet supported by scientific evidence.