An allergy is an immune response to a harmless exogenous substance. Allergic diseases, ranging from mild to life-threatening, affect between 15 and 20 percent of the world population. Although environmental factors such as certain bacterial and viral infections and cigarette smoke can contribute to the development of allergies, genetic factors play the biggest role. The Hygiene Hypothesis suggests that due to improved sanitation and healthcare, the normal early exposure to infections that is necessary for the development of a healthy immune system fails to occur, leading to the development of allergies. Some common allergic diseases are urticaria, atopic dermatitis, allergic rhinitis, allergic asthma, and food allergies.
Allergies are managed by rigorous attempts to avoid the allergen (the substance causing the allergy). Antihistamine drugs, especially non-sedating and long-lasting preparations, can help in prophylaxis against frequent attacks, while corticosteroids can be used to reduce inflammation when the attack has occurred. Some other drugs that can be used include sodium cromoglicate for prophylaxis against asthma and allergic rhinitis, and omalizumab for moderate to severe rhinitis and allergic asthma.