<unk>  is a disorder of the immune system that is often called atopy. <unk> c reactions occur to environmental substances known as<unk> ns; these reactions are acquired, predictable and rapid.  Strictly,<unk>  is one of four forms of hypersensitivity and is called type I (or immediate) hypersensitivity. It is characterized by excessive activation of certain white blood cells called mast cells and basophils by a type of antibody, known as IgE, resulting in an extreme inflammatory response.  Common<unk> c reactions include eczema, hives, hay fever, asthma, food<unk> es, and reactions to the venom of stinging insects such as wasps and bees.
Mild<unk> es like hay fever, are highly prevalent in the human population and cause symptoms such as<unk> c conjunctivitis, itchiness and runny nose. Similarly, conditions  such as asthma are common, in which<unk>  plays a major role. In some people, severe<unk> es to environmental or dietary<unk> ns, or to medication, occur that may result in life-threatening anaphylactic reactions and potentially death.
A variety of tests now exist to diagnose<unk> c conditions; these include testing the skin for responses to known<unk> ns or analyzing the blood for the presence and levels of<unk> n-specific IgE. Treatments for<unk> es include<unk> n avoidance, use of antihistamines, steroids or other oral medications, immunotherapy to desensitize the response to<unk> n, and targeted therapy.
Allergy
Hypersensitivity
Radiosensitivity
Photosensitivity
Autoimmunity
Anaphylaxis pathophysiology
Cross-reactivity
Envenomation