Mast Cell Activation Syndrome Overview: ABCs on MCAS

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taylor By Taylor A Doherty, MD
Associate Professor , Allergy & Immunology
UC San Diego School of Medicine

Classical allergies are manifested by symptoms of itching, watery eyes, nose stuffiness, sneezing, rashes and wheezing. Sometimes allergies cause life-threatening reactions that lower blood pressure and comprise breathing, especially if a patient is very sensitive to specific foods, medications or insect stings. Mast cells are the primary cell type that triggers these symptoms by producing histamine, prostaglandins, leukotrienes and many other substances. In the case of conventional allergies, mast cells are bound by a molecule called immunoglobulin E or “IgE”. Normally the body produces immunoglobulins to protect against foreign invaders such as bacteria and viruses. However, allergies can develop when IgE is produced against specific environmental agents that do not pose a threat to the host. Examples include pets, foods, medications and pollens.

Once the IgE is bound to the mast cell, the mast cell is a loaded gun that is ready to fire when the trigger is present in the blood and tissues. Medications including antihistamines, anti-leukotrienes (montelukast/Singulair), cromolyn, and corticosteroids are used to block this response and reduce symptoms. Doctors are also using an anti-IgE injection to help people with asthma and hives. Allergists commonly use allergy shots to change the way the immune system sees some allergens.

Newer to the world of allergy, Mast Cell Activation Syndrome (MCAS) is a disorder where mast cells are activated even without IgE or classic allergic triggering. Since mast cells are present in the gastrointestinal tract, lungs, nose/sinuses, blood vessels, nervous system, and skin, activated mast cells in any of these locations can produce a wide variety of symptoms. Skin flushing is common in MCAS and can occur along with hives, abdominal pain, diarrhea, lightheadedness, fainting, anxiety, brain fog, and many other symptoms. Patients with MCAS often have triggers that are not classic for other allergic patients and include environmental chemicals, temperature changes, many unrelated medications, alcohol, and unusual foods. Many times there are specific “flares” though some patients appear to have more chronic symptoms.

MCAS is diagnosed usually by an allergist/immunologist based on history and laboratory testing. Lab tests for MCAS include serum tryptase (mast cell product) and urine leukotrienes, histamine, and prostaglandins. Unfortunately, much of the testing is not well standardized as understanding MCAS is in its infancy. Positive testing is supportive of a MCAS diagnosis but negative testing is not unusual. Conventional therapy for MCAS includes H1 and H2 antihistamines, anti-leukotrienes, mast cell stabilizers (cromolyn, ketotifen), and corticosteroids. Anti-IgE appears to reduce mast cell responses even in patients without an IgE trigger and is being currently being tried in MCAS as well. Many patients with MCAS are able to control their symptoms with medication and trigger avoidance. Hopefully, with more understanding, novel therapies will be available to help those who still suffer with MCAS.

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