What is Mast Cell Activation Syndrome (MCAS)?

Mast Cell Activation Syndrome (MCAS) is a chronic condition where the body’s mast cells — part of the immune system — release too many chemicals at the wrong times. These chemicals, called mediators, normally help the body fight infections and heal injuries. In MCAS, they are released inappropriately, which can affect many different systems in the body.

Because of this, people with MCAS may experience allergic-type reactions such as flushing, hives, stomach pain, diarrhea, fatigue, brain fog, headaches, or even severe reactions like anaphylaxis. Symptoms vary widely from person to person, which makes MCAS challenging to diagnose and manage.

Raising awareness about MCAS is essential to improve diagnosis, support patients, and push forward research into better treatments.

What are Mediators?

Mediators are the chemical signals released by mast cells. They play an important role in how the immune system works — helping fight infections, heal wounds, and regulate inflammation.

Common Mast Cell Mediators

• Histamine – causes itching, flushing, hives, and stomach upset

• Tryptase – linked to inflammation and allergic reactions

• Prostaglandins & Leukotrienes – contribute to pain, swelling, and breathing issues

• Cytokines – immune messengers that affect many body systems

In MCAS, mast cells release these mediators too often or at the wrong times, leading to widespread symptoms that can affect the skin, gut, brain, lungs, and more.

What is a Mast Cell?

Mast cells are a type of immune cell found throughout the body — especially in the skin, lungs, gut, and around blood vessels.

They act like first responders for the immune system. When activated, mast cells release mediators (such as histamine, tryptase, and cytokines) that cause inflammation, help with healing, and protect the body.

In people with Mast Cell Activation Syndrome (MCAS), mast cells release mediators too often or at the wrong times, leading to allergic-type reactions and many different symptoms.