Allergy : An Overview
Salwa Hassan Teama






Allergy
Allergy refers to certain diseases in which immune responses to environmental antigens cause tissue inflammation and organ dysfunction. Hypersensitivity and sensitivity are synonyms for allergy.
Allergen is any antigen that causes allergy. The allergen is either inhaled or ingested and is then processed by the dendritic cell, an antigen-presenting cell. It can be complete protein antigens or low molecular weight proteins capable of eliciting an IgE response. Pollen and animal dander represent complete protein antigens.
Atopy is the inherited propensity to respond immunologically to such common naturally occurring allergens with continuous production of IgE antibodies.

Allergic Reaction Its overreaction to a harmless substance (an allergen)
This harmless substance that is contacted through the skin,
inhaled into the lungs, swallowed, or injected.
Types of Hypersensitivity Reaction
Hypersensitivity reactions require a pre-sensitized (immune) state of the host. Hypersensitivity reactions can be divided into four types: type I, type II, type III and type IV, based on the mechanisms involved and time taken for the reaction. Frequently, a particular clinical condition (disease) may involve more than one type of reaction.
Type I hypersensitivity is also known as immediate or anaphylactic hypersensitivity. The reaction may involve skin (urticaria and eczema), eyes (conjunctivitis), nasopharynx (rhinorrhea, rhinitis), bronchopulmonary tissues (asthma) and gastrointestinal tract (gastroenteritis). The reaction may cause a range of symptoms from minor to death. The reaction usually takes 15 - 30 minutes from the time of exposure to the antigen, although sometimes it may have a delayed onset (10 - 12 hours). Immediate hypersensitivity is mediated by IgE.
The primary cellular component in this hypersensitivity is the mast cell or basophil. The reaction is amplified and/or modified by platelets, neutrophils and eosinophils. A biopsy of the reaction site demonstrates mainly mast cells and basphil.

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