Allergy

An allergy occurs when the immune system overreacts to a normally harmless substance, treating it as a danger. This overreaction can lead to conditions like hay fever, allergic conjunctivitis, allergic asthma, atopic dermatitis, food allergies, and anaphylaxis. Symptoms vary depending on the allergen and can include red eyes, an itchy rash, sneezing, coughing, a runny nose, shortness of breath, or swelling. Common triggers include pollen, certain foods, metals, insect stings, medications, and materials like latex. Both genetic and environmental factors play a role in developing allergies.

The immune response involves immunoglobulin E (IgE) antibodies binding to the allergen, then attaching to receptors on mast cells or basophils. This triggers the release of inflammatory chemicals, such as histamine. Diagnosis usually starts with a person’s medical history; skin or blood tests may help in some cases, though a positive test does not always mean a significant allergy exists. Early exposure to potential allergens in childhood might offer some protection. Treatment focuses on avoiding known triggers and using medications like steroids or antihistamines. For severe reactions, injectable adrenaline (epinephrine) is recommended. Allergen immunotherapy, which involves gradually increasing exposure to an allergen, can help with certain allergies, such as hay fever and reactions to insect bites, but its effectiveness for food allergies is uncertain.

Allergies are widespread. In developed countries, around 20% of people have allergic rhinitis, food allergy affects 10% of adults and 8% of children, and about 20% have or have had atopic dermatitis at some point. Asthma rates vary by country, affecting 1–18% of people, while anaphylaxis occurs in 0.05–2% of the population. Rates of many allergic diseases appear to be rising. The term “allergy” was first introduced by Clemens von Pirquet in 1906.

Signs and symptoms depend on how the allergen enters the body. Airborne allergens like dust or pollen typically affect the eyes, nose, and lungs. Allergic rhinitis, or hay fever, causes sneezing, itching, and redness of the eyes, along with nasal irritation. Inhaled allergens can also increase mucus in the lungs, leading to shortness of breath, coughing, and wheezing. Allergic rhinitis often occurs alongside asthma. Food allergies can cause abdominal pain, bloating, vomiting, diarrhea, itchy skin, and hives, but rarely lead to respiratory symptoms. Insect stings, certain foods, antibiotics like penicillin, and medications such as aspirin can trigger a systemic reaction called anaphylaxis, which may affect the digestive, respiratory, and circulatory systems. Anaphylaxis can include skin reactions, bronchoconstriction, swelling, low blood pressure, coma, and death. It can start suddenly or be delayed, and symptoms may seem to fade but can return.

Skin contact with allergens like latex often leads to contact dermatitis or eczema, causing rashes, swelling, and inflammation. This can produce a “wheal and flare” reaction typical of hives and angioedema. Insect stings may cause a large local reaction—redness over 10 cm that lasts one to two days—which can also occur after immunotherapy. The skin normally blocks most allergens, but a sting can inject allergen into the skin, triggering a localized response. Mast cells in the skin activate, increasing blood vessel permeability and causing fluid leakage and swelling. This also stimulates the wheal-and-flare reaction, and nerve signals widen surrounding blood vessels, leading to redness. About eight hours after contact, a secondary, more widespread skin swelling may occur in some individuals. When an allergen is eaten, it can enter the bloodstream and reach the skin, causing a dispersed form of hives called urticaria. This skin reactivity allows allergists to test for allergies by injecting a tiny amount of allergen into the skin, which causes a local reaction—though even these small injections carry a risk of systemic anaphylaxis.

Airway contact with allergens can trigger reactions in the nose, sinuses, throat, and lungs. Allergic rhinitis, one of the most common airway allergies, is marked by sneezing, nasal congestion, runny nose, and itching of the nose and eyes. Common airborne triggers include pollen, house dust mites, animal dander, and mold spores. Reactions in the lower airways can contribute to allergic asthma, where exposure to allergens leads to breathing difficulties.