Allergy, Allergens and Allergenicity

An allergy is a hypersensitivity disorder of the immune system. The common symptoms include red eyes, itchiness and runny nose, eczema, hives or an asthma attack. In some people, severe allergies to environmental or dietary allergens or to medication may result in life-threatening reactions called “anaphylaxis”. Not all reactions or intolerances are forms of allergy. Allergic reactions occur when a person’s immune system reacts to normally harmless substances in the environment . A substance that causes a reaction is called an “allergen”. These reactions are acquired, predictable, and rapid. Allergy is one of four forms of hypersensitivity and is formally called “type I (or immediate) hypersensitivity”. Allergic reactions are distinctive because of excessive activation of certain white blood cells called “mast cells” and basophils by a type of antibody called “Immunoglobulin E (IgE)”. This reaction results in an inflammatory response which can range from uncomfortable to dangerous. A variety of tests exist to diagnose allergic conditions. If done they should be ordered and interpreted in light of a person’s history of exposure as many positive test results do not mean a clinically significant allergy. Tests include placing possible allergens on the skin and looking for a reaction such as swelling and blood tests to look for an allergen-specific IgE.

Signs and symptoms

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.
Inhaled allergens can also lead to asthmatic symptoms caused by narrowing of the airways(bronchoconstriction) and increased production of mucus in the lungs, shortness of breath(dyspnea), coughing and wheezing.
Allergic reactions can result from foods, insect stings and reactions to medications and antibiotics. Symptoms of food allergy include abdominal pain, bloating, vomiting, diarrhea, itchy skin and swelling of the skin during hives. Food allergies rarely cause respiratory(asthmatic) reactions or rhinitis.
Antibiotics and certain medicines produce a systemic allergic response called “anaphylaxis”, multiple organ systems can be affected, including the digestive system, the respiratory system and the circulatory system. Depending on the rate of severity, it can cause cutaneous reactions, bronchoconstriction, edema, hypotension, coma and even death. This type of reaction can be triggered suddenly or the onset can be delayed. The severity of this type of allergic response often requires injections of epinephrine, sometimes through a device known as the EpiPen or Twinject auto-injector. The nature of anaphylaxis is such that the reaction can seem to be subsiding, but may recur throughout a prolonged period of time.
Substances that come into contact with the skin such as latex, are also common causes of allergic reactions, known as contact dermatitis or eczema. Skin allergies frequently cause rashes or swelling and inflammation within the skin, in what is known as a “wheal and flare” reaction characteristic of hives and angioedema.


Risk factors for allergy can be placed in two general categories, host and environmental factors.
Host factors include heredity, sex, race and age, with heredity being by far the most significant.
Environmental factors include alterations in exposure to infectious diseases during early childhood, environmental pollution, allergen levels and dietary changes.


Treatments for allergies include avoiding known allergens, steroids that modify the immune system in general, and medications such as antihistamines and decongestants which reduce symptoms. Many of these medications are taken by mouth, although epinephrine, which is used to treat anaphylactic reactions, is injected. However, immunotherapy uses injected allergens to desensitize the body’s response.


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