Allergies are caused by adverse reactions from the immune system to particles, insect bites, or exposure to certain substances. These conditions are normally treated using antihistamines or corticosteroids. Antihistamine medications block the action of histamine which is released in response to allergens to prevent symptoms such as congestion, runny nose, itchiness, or swelling. Corticosteroids are steroid medications that prevent inflammation and swelling caused by allergies.