Mast cell activation syndrome (MCAS): Year of the Zebra 2026

Mast cell activation syndrome, or MCAS for short, is a spectrum of disorders affecting the function of mast cells, a group of immune cells that act as one of the body’s first responders. Mast cells are located strategically in places where the body interacts with the outside world, like the skin, lungs, and digestive tract. Inside mast cells, there are granules that contain chemical mediators like histamine and leukotrienes, which get released into the blood when they sense a potential threat, like an infection, an allergen, or some type of tissue injury.

These chemical mediators act like an alarm system that sets off the body’s inflammatory response. For example, histamine increases blood flow to the affected area by widening the blood vessels. It also makes the blood vessels more permeable so that immune cells can get there as soon as possible.

Other mediators, like leukotrienes, act on smooth muscle cells by tightening the airways or increasing gut mobility, all of which can help limit the spread of airborne or ingested threats. At the same time, they send out signals that recruit and activate other immune cells.

With MCAS, instead of responding only to real threats, mast cells activate in response to triggers that are not actually harmful, such as certain foods, fragrances, temperature changes, hormonal fluctuations, or even stress. Why this happens isn’t well understood, but some theories suggest mast cells become more sensitive due to changes in how they’re regulated, like differences in their surface receptors or internal signaling pathways. Sometimes, genetic factors or underlying conditions like chronic inflammation, infections, or autoimmune disorders play a role in making mast cells more reactive.

Now, because mast cells release chemical mediators that act throughout the body, MCAS can affect multiple organ systems at the same time. As a result, symptoms vary widely and often fluctuate in severity over time.

In the skin, individuals may experience flushing, itching, hives, or swelling. Digestive symptoms can include abdominal pain, nausea, diarrhea, or reflux. In the respiratory system, mast cell activation can lead to wheezing or shortness of breath due to airway constriction. The release of mediators can also cause blood vessels to dilate, leading to dizziness, lightheadedness, and a rapid heart rate, or even low blood pressure.