When the immune system overreacts to substances that are typically innocuous to most individuals, allergies—a common and complicated medical condition—occur. Individuals who are allergic or sensitive to these compounds, sometimes known as allergens, may experience an allergic reaction. Pollen, dust mites, pet dander, particular foods, insect stings, drugs, and even specific materials like latex can all be allergens.

When an allergic person comes into contact with an allergen, their immune system perceives it as a threat and releases a chemical called histamine, along with other chemicals and immune system cells. Histamine and these other substances cause a range of symptoms that characterize an allergic reaction.

The symptoms of allergies can vary depending on the specific allergen and the way it enters the body (inhalation, ingestion, or skin contact). Common allergic reactions include: