Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity of the immune system to something in the environment that usually causes little problem in most people. Common allergens include pollen and food. Metals and other substances may also cause problems. Food, insect stings, and medications are common causes of severe reactions. Their development is due to both genetic and environmental factors.
Many allergens such as dust or pollen are airborne particles. In these cases, symptoms arise in areas in contact with air, such as eyes, nose, and lungs. For instance, allergic rhinitis, also known as hay fever, causes irritation of the nose, sneezing, itching, and redness of the eyes. Symptoms of food allergy include abdominal pain, bloating, vomiting, diarrhea, itchy skin, and swelling of the skin during hives.
In the developed world, about 20% of people are affected by allergic rhinitis, about 6% of people have at least one food allergy, and about 20% have atopic dermatitis at some point in time. Depending on the country about 1 and 18% of people have asthma. Anaphylaxis occurs in between 0.05–2% of people. Rates of many allergic diseases appear to be increasing day by day. Immunotherapy is a preventive treatment for allergic reactions to substances such as grass pollens, house dust mites and bee venom.