MCAS and Lyme Disease: Symptoms, Rash and the Evidence

Quick answer

Mast cell activation syndrome (MCAS) causes repeated episodes of symptoms, such as flushing, hives, wheezing, belly cramps or low blood pressure, in two or more body systems at once. A link between MCAS and Lyme disease has been proposed but not proven. Alpha-gal syndrome, a red meat allergy linked to tick bites, is a separate and well-established condition.

What allergy experts say

A 2019 work group report from the American Academy of Allergy, Asthma & Immunology (AAAAI) describes three criteria for diagnosing MCAS:

Repeated episodes of symptoms consistent with mast cell activation, affecting two or more organ systems.
Lab evidence: a rise in a validated mast cell chemical, such as tryptase, measured during symptoms and compared with the person’s baseline.
Symptoms improve with medicines that block mast cell chemicals or stabilize mast cells.

The report warns that some publications and websites have broadened these criteria and relied on unvalidated tests, which has confused patients and clinicians.

What is mast cell activation syndrome?

Mast cells are immune cells found in many tissues, including the skin. When triggered, they release chemical messengers called mediators, which cause allergy-type reactions. In MCAS, these reactions happen in repeated episodes and look like a systemic allergic reaction (AAAAI).

The AAAAI report groups mast cell activation into types:

Primary. The mast cells themselves activate too easily. This includes systemic mastocytosis and clonal MCAS, often tied to changes in a gene called KIT.
Hereditary alpha-tryptasemia. An inherited trait involving extra copies of a tryptase gene. Some people with it have MCAS, but many do not.
Secondary. Normal mast cells are set off by an outside trigger, such as an allergen.
Idiopathic. MCAS with no trigger, gene change or inherited trait found.

MCAS symptoms

MCAS symptoms come in episodes and involve at least two body systems at the same time. The AAAAI report lists:

Body system	Examples of symptoms
Skin	Hives, itching, flushing, swelling (especially eyelids, lips and tongue)
Heart and blood vessels	Low blood pressure, fast heartbeat, fainting or near-fainting
Breathing	Wheezing, shortness of breath, noisy breathing in (stridor)
Digestive	Crampy belly pain, diarrhea, nausea, vomiting

The report also cautions that fatigue, headache or rashes on their own are too general to point to MCAS, and symptoms that are constant rather than episodic do not support the diagnosis.

MCAS rash vs Lyme rash vs alpha-gal reactions

Skin changes are one reason people wonder about MCAS and Lyme. The three look and behave differently:

	MCAS skin symptoms	Lyme rash (erythema migrans)	Alpha-gal reaction
What it looks like	Flushing, hives, itching, swelling	A red rash that slowly expands, up to 12 inches (30 cm) or more	Hives or itchy rash; swelling of lips, throat, tongue or eyelids
Itch	Can be itchy	May feel warm; rarely itchy or painful	Itchy
Timing	Comes in repeated episodes	Starts 3 to 30 days after a tick bite	Usually 2 to 6 hours after eating red meat or dairy or other alpha-gal products
Other symptoms	At least one other body system at the same time	May come with fever, chills, headache, fatigue and aches	May include nausea, belly pain, diarrhea, trouble breathing or a drop in blood pressure
Source	AAAAI	CDC	CDC

The Lyme rash appears in about 70 to 80% of people with Lyme (CDC). Flushing also has many other causes, including hormone-related conditions, certain tumors, medications and rosacea (AAAAI). See our Lyme rash guide and alpha-gal syndrome guide.

Is there a link between MCAS and Lyme disease?

What is known

Lyme bacteria can activate mast cells in the lab. In a 1999 study, Lyme bacteria caused rat and mouse mast cells in a dish to release small amounts of their contents and to make an inflammatory chemical called TNF-alpha (Talkington and Nickell). Mast cells live in the skin, where ticks pass on the bacteria.
Infection as a trigger is unclear. The AAAAI report lists infection only as a possible trigger, with inconclusive evidence.
Ticks can cause a real allergy: alpha-gal syndrome. Alpha-gal syndrome is a serious, potentially life-threatening allergy that can develop after tick bites, mainly from the lone star tick and less often from blacklegged ticks (CDC). CDC counted more than 110,000 suspected cases from 2010 to 2022 and says as many as 450,000 people may be affected. It is an allergy, not MCAS, and the lone star tick does not spread Lyme.

What is not known

We did not find clinical studies showing that Lyme disease causes MCAS in people. A lab dish result does not show that the same thing causes illness in the body. The AAAAI report also warns that many conditions have been wrongly blamed on MCAS without scientific support, and that some patients have received unproven and potentially harmful treatments as a result. Its biggest concern is missing another treatable condition.

If you have lingering symptoms after Lyme, MCAS is one of several possibilities people raise. It deserves a proper evaluation, not a label based on symptoms alone. See post-treatment Lyme disease syndrome and our Lyme and other conditions hub.

What patients tell us

We are collecting real experiences about MCAS, allergies and Lyme disease from people who have lived it, shared with their permission. As stories come in, they will appear here, clearly marked as personal experience rather than medical advice.

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Getting diagnosed: see an allergist or immunologist

Because the criteria are specific, MCAS is best evaluated by an allergist or immunologist. Steps may include:

Recording episodes: what happened, which body systems were involved, how long it lasted, and possible triggers.
Blood tests timed during symptoms, compared with a baseline level when you feel well.
Checking for other causes, such as allergies, hormone conditions, medications or skin conditions.
Testing for alpha-gal syndrome if you react after eating red meat or dairy. CDC says it is diagnosed with an exam, a detailed history and a blood test for alpha-gal antibodies, and managed with an allergist or other provider.

Questions to ask: Do my symptoms meet the consensus criteria? What else could explain them? Should I be tested for alpha-gal syndrome? Our doctor visit guide can help you prepare.

Emergency signs

Call 911 for signs of a severe allergic reaction (anaphylaxis): swelling of the lips, throat or tongue, trouble breathing, dizziness or fainting, or a drop in blood pressure. If you have been prescribed emergency medicine for allergic reactions, follow your provider’s plan.

Living with unexplained reactions is hard. Step 6 of our guide, living well with Lyme, has more support.

Frequently asked questions

What does an MCAS rash look like?

MCAS skin symptoms include flushing, hives, itching and swelling, especially of the eyelids, lips and tongue. They come in episodes along with symptoms in at least one other body system. A Lyme rash is different: it slowly expands at a tick bite and is rarely itchy.

Can Lyme disease cause MCAS?

It has not been shown. Lab studies found Lyme bacteria can activate mast cells in a dish, but we did not find clinical studies showing Lyme causes MCAS in people. Allergy experts warn against labeling symptoms as MCAS without meeting the criteria.

Is MCAS the same as alpha-gal syndrome?

No. Alpha-gal syndrome is an allergy to a molecule found in red meat and other mammal products, linked to tick bites, mainly from the lone star tick. MCAS is a mast cell disorder with its own criteria. Both should be evaluated by an allergist.

Are MCAS and POTS linked?

The AAAAI report says people with hereditary alpha-tryptasemia can have POTS and Ehlers-Danlos syndrome, but neither is caused by MCAS. See our page on Lyme and POTS.

Lyme Disease and Mast Cell Activation Syndrome (MCAS)