Allergen

An allergen is a substance that is usually harmless to most people. However, it can trigger an allergic reaction in sensitive individuals by stimulating an immune response. In technical terms, an allergen is an antigen that triggers a type-I hypersensitivity reaction. This occurs in atopic individuals through immunoglobulin E (IgE) responses. Most humans produce significant IgE responses only to defend against parasitic infections. In atopic people, non-parasitic antigens cause the body to produce IgE inappropriately.

The biological mechanism of an allergy involves a specific chain of events. When an allergen enters a predisposed person's body, it triggers the production of antibodies. These allergen antibodies migrate to mast cells. Mast cells line the moist membranes of the nose, eyes, and lungs. If the allergen enters the body again, the mast cells release chemicals called histamines. These histamines irritate and inflame the membranes. This process causes symptoms like a scratchy throat, itching, sneezing, and watery eyes.

Allergens can be categorized by how they enter the body. One method is ingestion, which involves consuming food or drink. Another is inhalation, such as breathing in pollen, perfume, or pet dander. Direct contact occurs when a body part brushes against an allergy-causing plant. Some people also experience reactions from insect stings, such as those from wasps, bees, or fire ants. Others may react to drugs like penicillin or substances like latex.

Food allergies are distinct from general food sensitivities. The United States Food and Drug Administration recognizes nine major food allergens. These include various items like peanuts, seafood, and milk. Different countries have different official lists due to genetic profiles and exposure levels. For example, Canada recognizes the nine US allergens plus mustard. The European Union also recognizes other organic grains.

Some reactions do not involve IgE antibodies. These are known as non-IgE-mediated or non-type-I hypersensitivity reactions. A common example is urushiol-induced contact dermatitis. Urushiol is a resin found in poison ivy and poison oak. This causes a type IV hypersensitivity reaction. It works by changing the configuration of a skin cell. This change makes the immune system no longer recognize the cell as part of the body. Other substances, like sawdust from wood products, can also cause skin rashes or asthma.

Seasonal allergies are often linked to the pollination cycles of plants. In the spring, trees like oak, elm, and maple pollinate. During the summer, grasses such as Bermuda and orchard grass pollinate. These allergies can cause rhinitis, which involves sneezing and a runny nose. They can also cause allergic conjunctivitis, which involves itchy, watery eyes. Some people may experience more severe symptoms like coughing, wheezing, or headaches. Genetics play a large role in these risks. If both parents have allergies, there is a 66% chance a child will have them. If only one parent has allergies, the risk is 60%.

Medical professionals use several methods to diagnose allergies. Doctors may perform skin tests by pricking the skin with purified allergen liquid. They can also inject a small amount of allergen under the skin. Blood tests, such as the radioallergosorbent test (RAST) or enzyme allergosorbent tests (EAST), detect high levels of IgE. In a laboratory, a doctor might examine a nasal smear for eosinophils. These are white blood cells that indicate an allergic condition. Other tools include imaging tests for sinusitis and nasal endoscopy, which uses a small camera to view nasal passageways.

Environmental changes are currently impacting allergy levels. Research suggests that global warming may worsen many allergy triggers. In the United States, 16 states have been named "Allergen Hotspots" due to increases in tree pollen. This is significant for the millions of people with asthma. In Canada, up to 75% of asthmatics also suffer from seasonal allergies. Additionally, fungal allergens like basidiospores are major airborne triggers. These spores come from mushrooms, rusts, and puffballs. Heavy rainfall can increase the release of these spores, which is linked to higher hospital admissions for children with asthma.