Mast Cell Activation Syndrome (MCAS): Symptoms, Causes, and Treatment Mast Cell Activation Syndrome (MCAS) is an immune system condition that can cause a wide range of symptoms affecting multiple organs throughout the body. Many patients experience symptoms for years before receiving a diagnosis because the condition can mimic allergies, gastrointestinal disorders, and autonomic nervous system conditions. At Galene Health Clinic , we frequently evaluate patients with complex