Allergen

An allergen is a substance that is usually harmless but can cause an allergic reaction in some people by triggering their immune response. These substances act as special signals called antigens that tell the body to react in a certain way.

Most of our bodies use a special kind of defense called immunoglobulin E, or IgE, to fight off parasites that can make us sick. But sometimes, people who are very sensitive to certain things, known as atopic individuals, their bodies make IgE even when there is no real danger. This wrong signal leads to what we call a type-I hypersensitivity reaction.

There are many different things that can become allergens for different people, and what causes a reaction in one person might not affect another. Groups like the World Health Organization and the International Union of Immunological Societies keep lists of all the substances that have been proven to cause these reactions in people.

Examples

Allergens can come from many places, like tiny bugs called dust mite or certain foods. Some foods, such as peanuts (a type of legume), nuts, seafood, and shellfish, can cause serious reactions in many people.

Different places have different rules about which foods can cause allergies. For example, the United States Food and Drug Administration has a list of major food allergens. Other countries may have different lists based on what people commonly eat there.

Allergic reactions can happen in many ways: eating a food you're sensitive to, breathing in pollen or perfume, or even touching a plant that causes allergies. Stings from wasp, fire ant, and bee, as well as medicines like penicillin and materials like latex, can also trigger reactions. A very serious reaction is called anaphylaxis, and one treatment is giving a medicine called epinephrine to help the body calm down.

Sometimes, a person can have an allergic reaction the very first time they come into contact with something, because their body has reacted to similar substances before.

Non-IgE

Some substances can cause reactions that are not related to the usual allergy process. For example, a substance in poison ivy and poison oak called urushiol can cause a rash on the skin. This happens because it changes the skin’s cells so the body sees them as foreign.

Even everyday things like trees, paper, or wood products can sometimes cause mild to severe reactions, like asthma or skin rashes, especially if you touch them or breathe in sawdust.

Common

Main article: List of allergens

Allergens are substances that can cause allergic reactions in some people. Besides proteins from blood transfusions and vaccines, common allergens include:

• Animal products like cat allergy to cats, fur, dander, cockroach calyx, wool, and dust mite excretion.
• Drugs such as penicillin, sulfonamides, and salicylates found in many fruits.
• Foods like celery, corn, eggs, various fruits, legumes including beans, peas, peanuts, and soybeans, milk, seafood, sesame, soy, tree nuts such as pecans and almonds, and wheat.
• Insect stings from bee and wasp venom, and mosquito bites.
• Mold spores.
• Metals like nickel and chromium.
• Other substances such as latex, wood, and plant pollens causing hay fever from grasses like ryegrass and timothy-grass, weeds like ragweed, and trees like birch.

Seasonal

Seasonal allergies happen at certain times of the year, especially in spring, summer, or fall, when plants like trees and grasses release pollen into the air. Different plants pollinate at different times—for example, some trees pollinate in spring, while certain grasses pollinate in summer. When someone is sensitive to these pollens, their body may react, causing symptoms like sneezing, a runny nose, itchy or watery eyes, and sometimes coughing or wheezing.

Doctors can help figure out if someone has seasonal allergies by looking at their symptoms and doing tests. These tests might include a blood test or a skin test to see how the body reacts to specific allergens. Other tools, like looking at cells from the nose under a microscope, can also help confirm an allergy. Some studies suggest that allergies might have developed as a way for the body to defend against parasites, and that changes in the climate may make allergies worse.

Fungal

Fungal spores from mushrooms, rusts, smuts, brackets, and puffballs can cause allergies, especially in people with asthma. These spores are found in the air and can trigger breathing problems. Rainfall often makes more spores appear, which can lead to more asthma cases in children.

Treatments for fungal allergies include medicines you can buy without a prescription, such as antihistamines and nasal sprays. These can help with sneezing and a stuffy nose. Some newer medicines cause less sleepiness. Allergy shots are also an option. These shots slowly give small amounts of the allergen to help the body get used to it. For egg and milk allergies, some children can slowly eat small amounts of cooked foods under the care of a dietitian to build tolerance.