Mast Cells

What are mast cells?

Mast cells are a part of your immune system. They’re made in your bone marrow, then move through your bloodstream into your tissues. The mature mast cells live in tissues throughout your body to help protect you from hazards around you. They can help fight infections and regulate your organs.

When your mast cells are overprotective, you may react to harmless things in your environment. This happens in people with seasonal allergies or allergic asthma. Mast cell activation can sometimes cause anaphylaxis, a serious allergic reaction.

What do mast cells do?

Mast cells act like an alert system. They protect your body from foreign invaders (pathogens like viruses, bacteria and parasites) and toxins (for instance, venom from snake bites or bee stings). They also fight off particles they think are harmful, even when they’re not. This causes allergic reactions.

Unlike many other types of white blood cells, they don’t destroy pathogens themselves. Instead, they sit in your tissues and wait for a message that something harmful is in your body.

Mast cell activation

Mast cells stand guard, ready to activate a quick response against invaders. They use surface proteins (receptors) that act like antennas to receive incoming immune signals. Other immune cells communicate which invaders to look out for, and sound an alarm if they find an intruder.

When your B cells (cells of your immune system) identify an invader, they become specialized to create lots of immunoglobulins (antibodies) to fight that particular invader. Immunoglobulin E (IgE) is the specific antibody made to alert the mast cells. IgEs quickly find and latch onto the IgE receptors on mast cells. The next time the same pathogen or allergen enters your body, the IgEs will catch it, like a fishing lure.

This will “switch on” (activate) the mast cells, which is kind of like pulling the fire alarm. But instead of fire doors closing, sprinklers spraying water and the fire department being alerted, the mast cells release histamine from their storage granules (degranulate). The shower of histamine causes a series of events that:

• Opens your blood vessels and breaks down connective tissue to allow immune cells to move into the area.
• Fill your tissues with fluid (swelling) to bring immune cells in and move the invaders out to your lymph nodes to be destroyed.
• Create mucus.
• Contract the smooth muscles that line your airways and gastrointestinal tract (stomach and intestines) in a wave-like motion (peristalsis) to push out harmful substances.

This can make your skin flushed, swollen or itchy. You might get a runny nose or sneeze and cough. You might even get diarrhea or vomit. In extreme cases, it can cause swelling of your airways and a life-threatening drop in blood pressure (severe anaphylaxis).

Mast cells and homeostasis

Mast cells also play a role in regulating your tissues and organs and keeping them in balance (homeostasis), including:

• Blood vessel growth.
• Bone growth.
• Cell growth and repair in your airways.

Where are mast cells found?

Unlike most of your white blood cells, very few mast cells are found in your bloodstream. You’re most likely to be exposed to something harmful by inhaling it, eating it or getting it in a break in your skin from injury. That’s why mast cells are mostly found in the tissues of your:

• Airways and lungs.
• Gastrointestinal (GI) tract.

They also surround your blood vessels and nerves.

What are the parts of a mast cell?

Mast cells contain a nucleus (the “brain” of the cell) surrounded by hundreds of granules (storage pouches). The granules contain chemical messengers (mediators) that send signals to your body in response to something that might harm you. These include:

• Histamine is a chemical signal that opens up your blood vessels to allow fluid and immune cells into your tissues. It also constricts your airways and GI tract and creates mucus, all in an effort to force out pathogens.
• Proteases (including tryptase, chymase and carboxypeptidase A). Proteases are enzymes that break down proteins. They help break down connective tissue so that immune cells can move into the area more easily.
• Cytokines/chemokines. These are small proteins that help signal immune cells to make more copies of themselves, make blood vessels “sticky” to grab immune cells and guide them into the tissues, and recruit cells to clean up and repair damage.
• Heparin prevents blood clots, which helps open your blood vessels.

Mast cell receptors

Mast cells have many receptors on their surface. These specially shaped proteins fit with other signaling proteins like a lock and key to activate the mast cells.

The most well-known mast cell receptor is the IgE receptor. Mast cells also have receptors for other antibodies (like immunoglobulin G or IgG) and ones that directly detect pathogens (toll-like receptors).

What are common mast cell disorders?

Some common mast cell disorders include:

• Environmental allergies.
• Asthma.
• Food allergies.
• Hives.

Some rare mast cell disorders include:

• Mast cell activation syndrome (MCAS). This is a condition where you experience repeated episodes of allergic or anaphylaxis symptoms without a known trigger.
• In this condition, too many mast cells build up in your body.
• Hereditary alpha-tryptasemia (or familial hypertryptasemia). This is a condition you’re born with that causes higher levels of tryptase (an enzyme mast cells produce) in your body. Most people with this genetic condition don’t have symptoms.

What are some common signs and symptoms of mast cell disorders?

The histamine released in mast cell disorders can lead to symptoms like:

• Discolored, hot or itchy skin.
• Swelling (edema).
• Shortness of breath.
• Episodes of anaphylaxis.

Common tests

If your provider thinks you have a mast cell disorder, they might check for signs of mast cell activation in your:

• Urine (pee).
• Blood.
• Bone marrow.
• Skin.

Your provider might do an allergy skin test or blood tests to identify triggers or look for signs of mast cell activation. A lab may do genetic testing to check for inherited conditions that might cause your symptoms.

What kind of treatments are there for mast cell disorders?

Treatments for mast cell disorders depend on your symptoms and what’s causing them. Treatments might include:

• Avoiding known triggers.
• Medications to manage symptoms. This might include antihistamines, acid reducers and leukotriene-modifying medications (like montelukast).
• Mast cell stabilizers. These medications prevent mast cells from degranulating.
• Medications to reduce or treat anaphylactic episodes. This includes epinephrine and omalizumab.
• Cytoreductive therapy/chemotherapy. Providers sometimes use chemotherapy medications (typically used to kill cancer cells) to treat aggressive mastocytosis.

Are mast cells good or bad?

Since most people have never heard of mast cells until they’re making you sick, you might think mast cells are bad. But they actually help defend your body against harmful invaders and regulate your organs. So, most of the time, they’re good.

A note from Cleveland Clinic

Sirens are going off. Security’s on the scene. Your mast cells have sounded the alarm, and your immune system is rushing in to save the day. They’re a vital part of our body’s defense system, but you usually only notice mast cells when they’re a little too over-enthusiastic about their job. But they strengthen our defenses and help regulate our blood vessels and organs. So next time you sniffle, sneeze or itch, thank your mast cells for their vigilance.