Mast cell activation syndrome (MCAS) occurs when the mast cells in your body release too much of the substances inside them at inappropriate times.

Mast cells are part of your immune system. They’re found in your bone marrow and around the blood vessels in your body. When you’re exposed to stress or danger, your mast calls respond by releasing substances called mediators. Mediators cause inflammation, which helps your body heal from an injury or infection.

This same response happens during an allergic reaction. Your mast cells release mediators to remove the thing you’re allergic to. For example, if you’re allergic to pollen, your mast cells release a mediator called histamine, which makes you sneeze to get rid of the pollen.

If you have MCS, your mast cells release mediators too frequently and too often. It’s different from another mast cell disorder called mastocytosis, which happens when your body makes too many mast cells.

Keep reading to learn more about MCAS, which, according to , is becoming more common.

Too many mediators can cause symptoms in almost every system in your body. However, the most commonly affected areas include your skin, nervous system, heart, and gastrointestinal tract. Depending on how many mediators are released, your symptoms can range from mild to life-threatening.

Symptoms you might experience in different parts of your body include:

  • skin: itching, flushing, hives, sweating
  • eyes: itching, watering
  • nose: itching, running, sneezing
  • mouth and throat: itching, swelling in your tongue or lips, swelling in your throat that blocks air from getting to your lungs
  • lungs: trouble breathing, wheezing
  • heart and blood vessels: low blood pressure, rapid heart rate
  • stomach and intestines: cramping, diarrhea, nausea, abdominal pain
  • nervous system: headache, dizziness, confusion, extreme tiredness

In severe cases, you can develop a life-threatening condition called anaphylactic shock. This causes a rapid drop in your blood pressure, a weak pulse, and narrowing of the airways in your lungs. It usually makes it very hard to breath and requires emergency treatment.

Researchers aren’t sure what causes MCAS. However, a 2013 study noted that of participants with MCAS had at least one first-degree relative who also had it. This suggests that there’s likely a genetic component to MCAS.

Episodes of MCAS are always triggered by something, but it can be hard to figure out what the trigger is.

Some common triggers include:

  • allergic-type triggers, such as insect bites or certain foods
  • drug-induced triggers, such as antibiotics, ibuprofen, and opiate pain relievers
  • stress-related triggers, such as anxiety, pain, rapid temperature changes, exercise, being overly tired, or an infection
  • smells, such as perfume
  • hormonal changes, such as those related to a woman’s menstrual cycle
  • mast cell hyperplasia, which is a rare condition that can occur with some cancers and chronic infections

When your doctor can’t find a trigger, it’s called idiopathic MCAS.

MCAS can be hard to diagnose because its symptoms overlap with those of many conditions.

To be diagnosed with MCAS, you must meet the following criteria:

  • Your symptoms affect at least two body systems and are recurrent, and there’s no other condition causing them.
  • Blood or urine tests performed during an episode show you have higher levels of markers for mediators than you do when you aren’t having an episode.
  • Medications that block the effects of the mast cell mediators or their release make your symptoms go away.

Before diagnosing your condition, your doctor will review your medical history, give you a physical exam, and run blood and urine tests to rule out any other causes of your symptoms. They may also have you avoid certain foods or medications for a period of time to narrow down what your triggers might be.

They may also ask you to keep a detailed log of your attacks, including any new foods you ate or medications you took before it started.

There’s no cure for MCAS, but there are ways to manage your symptoms. Mild symptoms, such as headaches, usually respond well to over-the-counter pain relievers, such as ibuprofen (Advil).

For more moderate symptoms, you may need:

  • H1 or H2 antihistamines. These block the effects of histamines, which are one of the main mediators released by mast cells
  • Mast cell stabilizers. These prevent the release of mediators from mast cells.
  • Antileukotrienes. These block the effects of leukotrienes, another common type of mediator.

For severe symptoms, such as anaphylactic shock, you’ll need an injection of epinephrine. This can be done at a hospital or with an auto injector (EpiPen). Consider wearing a medical ID bracelet if your symptoms are severe, at least until you figure out what your triggers are.

Although it’s an uncommon condition, MCAS can produce distressing symptoms that interfere with your daily life. However, with proper diagnosis and treatment, your symptoms can be controlled. In addition, once you know what factors triggers an episode, you can avoid them and reduce the number of episodes you have.