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This article deals specifically with IgE mediated hypersensitivity. For other types of allergic or hypersensitive reactions see the main article: Hypersensitivity

An allergy or Type I hypersensitivity is a immune malfunction whereby a person's body is hypersensitised to react immunologically to typically nonimmunogenic substances. 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 "reaction" or "reactivity". Type I hypersensitivity is characterised by excessive activation of mast cells 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.

1 History

The term and concept of "allergy" were coined by Viennese pediatrician Baron 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.

2 Signs and symptoms

Allergy is characterised by a local or systemic inflammatoryInflammation is the first response of the immune system to infection or irritation and may be referred to as the innate cascade. Inflammation is characterized by the following quintet: redness rubor , heat calor , swelling tumor , pain dolor and dysfuncti response to allergens. Local symptoms are:

• Nose: swelling of the nasal mucosa ( allergic rhinitis)
• Eyes: redness and itching of the conjunctivaThe conjunctiva is a membrane that covers the sclera (white part of the eye) and lines the inside of the eyelids. It helps lubricate the eye by producing mucus and tears, although not as much tears as the lacrimal gland. London Place Eye Center, 2003). ( allergic conjunctivitis )
• Airways: bronchoconstriction , wheezing and dyspnoea, sometimes outright attacks of asthmaAsthma is an immunological disease which causes difficulty in breathing. It is a form of type I hypersensitivity in which the bronchioles in the lungs are narrowed by inflammation and spasm of the lining of the airway wall. A person with asthma may experi
• Skin: various rashA rash is a change in the skin which affects its appearance or texture. A rash may be localised to one part of the body, or affect all the skin. Rashes may cause the skin to change colour, itch, become warm, bumpy, dry, cracked or blistered, swell and mayes, such as eczema, hives ( urticaria) and contact dermatitis.

Systemic allergic response is also called anaphylaxis. Depending of the rate of severity, it can cause cutaneous reactions, bronchoconstriction, oedema, hypotension, coma and even death.

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