Allergy

An allergy or Type I hypersensitivity is an 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.

Pathophysiology

All hypersensitivities result from an aberration somewhere in the normal immune process. However the exact cause of such malfunctions are not always been apparent, and several arguments from genetic-basis, environmental-basis and intermediate proponents exist with varying validity and acceptance.

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Acute response

The difference between a type I hypersensitivity reaction against an allergen to the normal humoral response against a foreign body is that plasma cells secrete IgE as opposed to either IgM (against novel antigens) or IgG (against immunized antigens). IgE binds to Fc receptors on the surface of mast cells and basophils, both involved in the acute inflammatory response.

Related Topics:
Humoral response - Plasma cell

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When IgE is first secreted it binds to the Fc receptors on a mast cell or basophil, and such a IgE-coated cell is said to be sensitized to the allergen in question. A later exposure by the same allergen causes reactivation of these IgE, which then signal for the degranulation of the sensitized mast cell or basophil. These granules release histamine and other inflammatory chemical mediators (cytokines, interleukins, leukotrienes, and prostaglandins) into the surrounding tissue causing several systemic effects, such as vasodilation, mucous secretion, nerve stimulation and smooth muscle contraction. This results in the previously described symptoms of rhinorrhea, itchiness, dyspnea, and anaphylaxis. Depending on the individual, allergen, and mode of introduction, the symptoms can be system-wide (calliscal anaphylaxis), or localised to particular body systems (for example, asthma to the respiratory system; eczema to the dermis).

Related Topics:
Degranulation - Histamine - Cytokine - Interleukin - Leukotriene - Prostaglandin - Vasodilation - Mucous - Nerve - Smooth muscle

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Late-phase response

After the chemical mediators of the acute response subside, late phase responses can often occur. This is due to the migration of other leukocytes such as neutrophils, lymphocytes, eosinophils and macrophages to the initial site. The reaction is usually seen 4-6 hours after the original reaction and can last from 1-2 days. Cytokines from mast cells may also play a role in the persitence of long-term effects. Late phase responses seen in asthma are slightly different to those seen in other allergic responses.

Related Topics:
Leukocyte - Neutrophil - Lymphocyte - Eosinophil - Macrophage - Asthma

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