Definition of Allergy
An allergy can refer to several kinds of immune reactions including Type I hypersensitivity in which a person's body is hypersensitised and develops IgE type antibodies to typical proteins. When a person is hypersensitised, these substances are known as allergens. The word allergy derives from the Greek words allos meaning "other" and ergon meaning "work". Type I hypersensitivity is characterised by excessive activation of mast cells and basophils by immunoglobulin E resulting in a systemic inflammatory response that can result in symptoms as benign as a runny nose, to life-threatening anaphylactic shock and death.
Allergy is a very common disorder and more than 50 million Americans suffer from allergic diseases. Allergies are the 6th leading cause of chronic disease in the United States, costing the health care system $18 billion annually.
The term and concept of "allergy" was coined by a Viennese pediatrician named Clemens von Pirquet in 1906. He observed that the symptoms of some of his patients might have been a response to outside allergens such as dust, pollen, or certain foods. For a long time all hypersensitivities were thought to stem from the improper action of inflammatory immunoglobulin class IgE, however it soon became clear that several different mechanisms utilizing different effector molecules were responsible for the myriad of disorders previously classified as "allergies". A new four-class (now five) classification scheme was designed by P. G. H. Gell and R. R. A. Coombs. Allergy has since been kept as the name for Type I Hypersensitivity, characterised by classical IgE mediation of effects.
Signs and Symptoms
Allergy is characterised by a local or systemic inflammatory response to allergens. Local symptoms are:
- Nose: swelling of the nasal mucosa (allergic rhinitis)
- The distinctive behavior known as nasal salute, also known as allergy salute, is the habit of wiping of the nose in an upward direction due to itching.
- Eyes: redness and itching of the conjunctiva (allergic conjunctivitis)
- Airways: bronchoconstriction, wheezing and dyspnoea, sometimes outright attacks of asthma
- Ears: feeling of fullness, possibly pain, and impaired hearing due to the lack of eustachian tube drainage.
- Skin: various rashes, such as eczema, hives (urticaria) and contact dermatitis.
- Head: while not as common, headaches are seen in some with environmental or chemical allergies.
Systemic allergic response is also called anaphylaxis. Depending of the rate of severity, it can cause cutaneous reactions, bronchoconstriction, edema, hypotension, coma and even death.
Hay fever is one example of an exceedingly common minor allergy — large percentages of the population suffer from hayfever symptoms in response to airborne pollen. Asthmatics are often allergic to dust mites. Apart from ambient allergens, allergic reactions can be due to medications.