Allergy is a reaction of the immune system to a foreign substance that does not bother 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 immune systems reaction can inflame the skin, sinuses, airways, or digestive system, and the severity of allergies can vary from person to person, ranging from minor irritation to anaphylaxis, which is a potentially life-threatening emergency.

Allergens can be found in various substances, including pollen, animal dander, dust mites, mold, certain foods, insect stings, medications, and latex. People who have a family history of asthma or allergies, such as hay fever, hives, or eczema, are more likely to develop allergies. Allergy can cause symptoms in different parts of the body, such as the nose and/or eyes, resulting in allergic rhinitis (hay fever) and/or conjunctivitis, skin resulting in eczema or hives, and lungs resulting in asthma.

When a person who is allergic to a particular allergen comes into contact with it, an allergic reaction occurs. The allergen triggers an antibody response, and the antibodies attach themselves to mast cells. When the allergen comes into contact with the antibodies, the mast cells respond by releasing histamine, which causes inflammation, redness, and swelling. Allergies can be treated with medications such as antihistamines, decongestants, nasal steroids, asthma medicines, and immunotherapy.