An allergy is an immune system response to a foreign substance that's not typically harmful to your body. These foreign substances are called allergens. They can include certain foods, pollen, or pet dander. Your immune system's job is to keep you healthy by fighting harmful pathogens. An allergic reaction occurs when cells in the immune system interpret a foreign substance or allergen as harmful. The immune system overreacts to these allergens and produces histamine, which is a chemical that causes allergy symptoms, such as inflammation, sneezing, and coughing. When a harmless substance such as dust, mold, or pollen is encountered by a person who is allergic to that substance, the immune system may overreact by producing antibodies that "attack" the allergen. The can cause wheezing, itching, runny nose, watery or itchy eyes, and other symptoms. Over-the-counter (OTC) antihistamines and decongestants may relieve minor symptoms of an allergic reaction. Antihistamines prevent symptoms such as hives by blocking histamine receptors, so your body doesn't react to the allergens. Decongestants help clear your nose and are especially effective for seasonal allergies.