Allergy is an abnormal immune system reaction to a substance foreign to the body that is acquired, predictable and rapid. The term "allergy" is often used for other abnormal reactions to substances. Type I hypersensitivity is characterized by excessive activation of mast cells and basophils by IgE (Immunoglobulin E), resulting in an inflammatory response. Mild allergies (such as hay fever, leading to symptoms such as allergic conjunctivitis / red eyes caused by allergies, and runny nose) are highly prevalent, as are conditions in which allergy plays a major role (such as asthma); some people have more severe allergies to environmental or dietary allergens (or medication) that may result in life-threatening anaphylactic reactions and potentially death. Allergic conditions can be diagnosed in various ways, such as skin or blood tests. Treatment may include allergen avoidance, antihistamines, various other oral medications, immunotherapy (desensitization) and targeted therapy.