An allergy is a reaction of the immune system to a substance in the environment that is usually harmless to most people, known as an allergen. When a person with allergies comes into contact with an allergen, their immune system produces antibodies that identify the allergen as harmful, even though it is not. The antibodies attach themselves to mast cells, which release histamine when the allergen comes into contact with the antibodies. This histamine release causes inflammation, which can lead to a range of symptoms depending on the type and severity of the allergy.

Common allergens include pollen, animal dander, dust mites, mold, certain foods (such as peanuts, tree nuts, wheat, soy, fish, shellfish, eggs, and milk), insect stings, medications (particularly penicillin or penicillin-based antibiotics), and latex or other substances that can cause allergic skin reactions.

Symptoms of an allergic reaction can range from mild to severe and can include:

• Congestion, runny nose, and sneezing
• Itchy, watery eyes
• Skin rash or hives
• Stomach cramps, diarrhea, or vomiting
• Swelling of the lips, tongue, or throat
• Difficulty breathing or wheezing
• Anaphylaxis, a potentially life-threatening emergency.

Risk factors for developing allergies include having a family history of asthma or allergies, being a child, having asthma or another allergic condition, and exposure to certain environmental factors. Allergies can be managed with medications such as antihistamines, decongestants, nasal steroids, and asthma medicines, as well as immunotherapy.