An allergic reaction is a reaction to a substance that is normally harmless to most other people. Allergies happen when a person’s immune system overreacts to a normally harmless substance that the person has breathed in, touched, or eaten. Allergens— the antigens that bring on an allergic reaction—may be foods, medications, plants or animals, chemicals, dust, or molds. Some common allergic reactions are hay fever, allergic conjunctivitis (an eye reaction); asthma, pet-dander allergies, and skin reactions, such as hives.
A common cause for allergies are dust mites, a large part of household dust. If they are breathed in by an allergic person, the body parts of the dead mites can trigger asthma, a lung condition that causes a person to have difficulty breathing. Cat and dog dander, or skin flakes, can cause an allergic reaction, such as sneezing, wheezing, and running eyes and nose. Common food allergy triggers are the proteins in cow’s milk, eggs, peanuts, wheat, soy, fish, shellfish, and tree nuts.