AllergyNest

Allergy glossary

These are the words that come up most on AllergyNest’s pollen, allergen, symptom and condition pages, each explained in a sentence or two. Every definition names the source it comes from, and where AllergyNest has a fuller page on a term, a link leads to it.

Editor: Ara OhanianUpdated

A

Advisory statements
Statements on food labels such as “may contain” or “produced in a facility that also uses” an allergen. FDA says they are not required by law and must be truthful and not misleading.26 See Food allergen labels.
Air Quality Index (AQI)
Numbers from 0 to 500 used to tell people about the air. If the air is dirtier, the numbers get bigger, and on days measuring more than 100, the air can be bad to breathe.1
Allergen
An ordinarily harmless substance that causes an abnormal reaction in a person with an allergy. Allergens can include dust mites, mold, and tree, weed and grass pollen, as well as food proteins such as milk, egg, soy, wheat, nut or fish.2 See allergens A to Z.
Allergic reaction
A reaction that occurs when the body mistakenly recognizes a common allergen, such as pollen or a food, as a threat and produces IgE antibodies, which travel to cells that release chemicals. It usually causes symptoms in the nose, lungs, throat or on the skin.3 See allergy symptoms.
Allergist
Also called Allergist/immunologist. A specialist with advanced training and experience to determine what is causing allergy symptoms.2
Allergy
A chronic condition involving an abnormal reaction to an ordinarily harmless substance called an allergen. The immune system views the allergen as an invader, and a chain reaction begins.2 See allergens A to Z and allergic conditions.
Alpha-gal syndrome
Also called AGS, Red meat allergy and Tick bite meat allergy. A serious, potentially life-threatening allergy and tickborne disease. Alpha-gal is a molecule found in most mammals, such as cows and pigs, but not in people, and symptoms occur after people eat red meat or are exposed to other products made from mammals.23 See Alpha-gal syndrome.
Anaphylaxis
A serious allergic reaction. It can begin very quickly, and its symptoms may be life-threatening.29
Angioedema
Swelling of the deeper layers of the skin that often occurs with hives. It is usually not red or itchy and often involves the eyelids, lips, tongue, hands or feet.12 See Angioedema and Can allergies cause swollen lips or a swollen face?.
Antibody
In an allergy, white blood cells of the immune system produce IgE antibodies, which attach themselves to special cells called mast cells, causing a release of potent chemicals such as histamine.2
Asthma
A chronic condition that affects the airways in the lungs, the tubes that carry air in and out of the lungs.31 See Allergic asthma.
Atopy
The genetic tendency to develop allergic diseases such as allergic rhinitis, asthma and atopic dermatitis (eczema). It is typically associated with heightened immune responses to common allergens, especially inhaled and food allergens.4 See Eczema (atopic dermatitis).

B

Blood test for allergies
Also called Specific IgE (sIgE) blood test. An allergy test that tells whether a person makes antibodies to an allergen. As with skin testing, a positive blood test to an allergen does not necessarily mean that the allergen caused symptoms.19

C

Cedar fever
An allergic reaction to the pollen released by mountain cedar trees; in Texas, the predominant species of mountain cedar is the Ashe juniper. The trees’ pollination period falls in the middle of flu season.32 See Cedar fever and Juniper and cedar pollen allergy.
Contact dermatitis
A skin condition caused by contact with a biological or chemical material. In the allergic form, contact with a specific material triggers an immune response; in the irritant form, the material damages the skin where it touches.12 See Contact dermatitis.
Cross-contact
The inadvertent introduction of a major food allergen into a product. It can occur when foods with different allergen profiles are produced in the same facility using shared equipment or on the same production line.26 See Food allergen labels.
Cross-reactivity
When the proteins in one substance, typically pollen, are similar to the proteins found in another, typically a food. For example, a person allergic to birch tree pollen may find that eating apples causes a reaction.5 See Oral allergy syndrome (pollen food allergy syndrome).

D

Dander
A pet’s dead skin cells. A pet allergy is a reaction to a protein found in the animal’s dander, saliva and urine.21 See Cat allergy and Dog allergy.
Dust mites
Tiny creatures, too small to be seen without a microscope or magnifying glass, that frequently make their home in places like furniture, carpets and bedding. They have eight legs, are relatives of spiders rather than true insects, and feed on flakes of skin shed by people and animals.14 See Dust mite allergy.

E

Eczema
Also called Atopic dermatitis. A chronic skin condition that usually begins in infancy or early childhood and shows as a scaly, itchy rash. It is often associated with other allergic diseases, such as food allergy, allergic rhinitis and asthma.12 See Eczema (atopic dermatitis).

F

Food allergy
A reaction that occurs when the immune system overreacts to a food or a substance in a food.15 See /allergies/food.
Food intolerance
Also called Food sensitivity. Difficulty digesting a particular food, which can lead to symptoms such as intestinal gas, abdominal pain or diarrhea. Food intolerances involve the digestive system, while food allergies involve the immune system.6 See Can a food allergy upset your stomach?.

H

Hay fever
Also called Allergic rhinitis. A group of symptoms affecting the nose. Seasonal symptoms usually come from airborne mold spores or tree, grass and weed pollen, and year-round symptoms generally from dust mites, pet hair or dander, cockroaches or mold.16 See Hay fever (allergic rhinitis).
Histamine
A chemical released by mast cells in the immune system when the body encounters an allergen. Its release causes an allergic reaction.7
Hives
Also called Urticaria. Red, itchy, raised areas of the skin that can range in size and appear anywhere on the body.12 See Can allergies cause hives?.

I

IgE
Also called Immunoglobulin E. Antibodies produced by the immune system. In an allergy, the body mistakenly recognizes a common allergen, such as pollen or a food, as a threat and produces IgE, and each type of IgE specifically targets a certain allergen.3
Immune system
A network of cells, organs and proteins that work together to defend and protect the body from “invaders” such as viruses, infections and disease.8

L

Latex allergy
An overreaction of the immune system to proteins in natural rubber latex, which is used to make some gloves, condoms, balloons, rubber bands, erasers and toys. Exposure often causes contact dermatitis symptoms; in some people, latex allergy can trigger anaphylaxis.9 See Latex allergy and Latex-fruit syndrome.

M

Major food allergens
The nine foods US law names as major food allergens: milk, eggs, fish, crustacean shellfish, tree nuts, peanuts, wheat, soybeans and sesame. Sesame was added as the ninth, effective January 1, 2023.26 See Food allergen labels and /allergies/food.
Mast cell
A special cell that IgE antibodies attach to in an allergy, causing a release of potent chemicals such as histamine.2
Mold
A type of fungus that produces spores that float through the air. Outdoor molds often cause allergy symptoms in summer and fall.20 See /allergies/mold and Mold spore counts.
Mountain cedar
Also called Ashe juniper and Juniperus ashei. A small, native, evergreen juniper tree or shrub. Its main population occurs in west-central Texas, largely on the Edwards Plateau.27 See Juniper and cedar pollen allergy and Cedar fever.

N

National Allergy Bureau
Also called NAB. The part of the American Academy of Allergy, Asthma & Immunology’s Aeroallergen Network that reports current pollen and mold spore levels from counting stations throughout the United States.13 See Pollen index vs. count and Mold spore counts.

O

Oral allergy syndrome
Also called OAS, Pollen food allergy syndrome and PFAS. A reaction that happens because proteins in some fruits and vegetables are very similar to those in particular pollens. These proteins can confuse the immune system in people with food or outdoor allergies.10 See Oral allergy syndrome (pollen food allergy syndrome).
Ozone
A gas made of three atoms of oxygen. Ground-level ozone forms when nitrogen oxides and volatile organic compounds react in sunlight, and breathing it can harm health.24

P

Particulate matter (PM2.5)
Also called Particle pollution. A mixture of solid particles and liquid droplets found in the air. PM2.5 means fine inhalable particles with diameters generally 2.5 micrometers and smaller; the average human hair is about 70 micrometers across.25
Patch test
A test that determines what allergen may be causing contact dermatitis. A small amount of a possible allergen is placed on the skin, and a sensitivity to it causes a local rash.19 See Contact dermatitis.
Pollen
The male fertilizing agent of flowering plants, trees, grasses and weeds. It is also a major allergen that causes symptoms of seasonal allergic rhinitis (hay fever).11 See pollen by metro.
Pollen count
The number of grains of pollen in a cubic meter of air. North Carolina’s Division of Air Quality, for example, collects each sample for 24 hours, and its laboratory technicians analyze the pollen under a microscope.30 See Pollen index vs. count.
Pollen index
Many different local pollen indices are used worldwide. The Universal Pollen Index is designed to offer a globally unified scale for comparing pollen levels across different areas.28 See Pollen index vs. count and how our pollen index works.
Pollen season
Each spring, summer and fall, and even in winter in some states, plants release tiny pollen grains. In the US, pollen seasons run from February to April for trees (some regions start as early as December or January), April to early June for grasses, and August to the first hard frost for weeds.22 See /allergies/tree-pollen, /allergies/grass-pollen and /allergies/weed-pollen.
Postnasal drip
Mucus that runs from the back of the nose down the throat. It is one of the symptoms of pollen allergy.22 See Can allergies cause postnasal drip?.

R

Ragweed
A weed, with 17 species in the United States, that grows in most regions, typically blooming and producing a fine-powder pollen from August into November. Its pollen usually reaches peak levels in mid-September.17 See Ragweed pollen allergy.

S

Sensitization
Also called Sensitivity. When the immune system makes IgE antibodies to a substance. A person may test positive to an allergen but not have an allergy.19
Skin prick test
Also called Scratch test. An allergy test in which a small drop of the allergen in question is pricked or scratched on the skin. A positive result does not necessarily mean an allergy.19
Spore
The “seed” of a mold, which travels through the air. Upsetting a mold source can send spores into the air.20 See Mold spore counts and /allergies/mold.

U

Universal Pollen Index
Also called UPI. An index in Google’s Pollen API, designed to offer a globally unified scale for comparing pollen levels across different areas. Its six categories run from 0 to 5: None, Very Low, Low, Moderate, High and Very High.28 See how our pollen index works.

W

Wheezing
A whistling or squeaky sound in the chest when breathing, especially when breathing out.18 See Can allergies cause wheezing?.
Wind-pollinated plants
Plants such as trees, grasses and weeds that make small, light, dry pollen grains that travel by the wind. Most of the pollen that causes allergic reactions comes from these plants, while flowering plants that spread their pollen by insects do not cause allergic rhinitis.22 See /allergies/tree-pollen, /allergies/grass-pollen and /allergies/weed-pollen.

Sources

  1. What Is the AQI?. AirNow, U.S. Environmental Protection Agency. Read on October 17, 2026.
  2. Allergy Defined. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  3. Immunoglobulin E (IgE) Defined. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  4. Atopy Defined. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  5. Cross-Reactivity Defined. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  6. Food Intolerance Defined. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  7. Histamine Defined. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  8. Immune System Defined. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  9. Latex Allergy Defined. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  10. Oral Allergy Syndrome (OAS) Defined. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  11. Pollen Defined. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  12. Allergic Skin Conditions. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  13. Controlling Hay Fever Symptoms with Accurate Pollen Counts. American Academy of Allergy, Asthma & Immunology. Read on October 17, 2026.
  14. Dust Allergies. American College of Allergy, Asthma and Immunology. Read on October 17, 2026.
  15. Food Allergy. American College of Allergy, Asthma and Immunology. Read on October 17, 2026.
  16. Hay Fever (Rhinitis). American College of Allergy, Asthma and Immunology. Read on October 17, 2026.
  17. Ragweed Allergy. American College of Allergy, Asthma and Immunology. Read on October 17, 2026.
  18. Wheezing, Shortness of Breath. American College of Allergy, Asthma and Immunology. Read on October 17, 2026.
  19. Allergy Diagnosis. Asthma and Allergy Foundation of America. Read on October 17, 2026.
  20. Mold Allergy. Asthma and Allergy Foundation of America. Read on October 17, 2026.
  21. Allergic to Your Pet? Learn About Dog and Cat Allergies. Asthma and Allergy Foundation of America. Read on October 17, 2026.
  22. Pollen Allergies. Asthma and Allergy Foundation of America. Read on October 17, 2026.
  23. About Alpha-gal Syndrome. Centers for Disease Control and Prevention. Read on October 17, 2026.
  24. Ground-level Ozone Basics. U.S. Environmental Protection Agency. Read on October 17, 2026.
  25. Particulate Matter (PM) Basics. U.S. Environmental Protection Agency. Read on October 17, 2026.
  26. Food Allergies. U.S. Food and Drug Administration. Read on October 17, 2026.
  27. Juniperus ashei. USDA Forest Service, Fire Effects Information System. Read on October 17, 2026.
  28. Pollen Index. Google Maps Platform, Pollen API documentation. Read on October 17, 2026.
  29. Anaphylaxis. MedlinePlus, U.S. National Library of Medicine. Read on October 17, 2026.
  30. Pollen Monitoring. North Carolina Department of Environmental Quality, Division of Air Quality. Read on October 17, 2026.
  31. What Is Asthma?. National Heart, Lung, and Blood Institute. Read on October 17, 2026.
  32. Cedar Fever – Explained. Texas A&M Forest Service. Read on October 17, 2026.

Definitions drafted with AI help from the sources above, then checked against them in separate AI fact-check passes. How we write these pages. Not reviewed by a clinician. General information, not medical advice.