Immediate hypersensitivity

This type of reaction is caused by "reagin" antibodies, which consist mainly of lgE, that react with allergens such as housedust mite, animal dander, or grass pollens. These reactions may occur in both the skin or the lung to produce asthma. Allergic reactions to insect stings can cause severe systemic effects—"anaphylaxis", which literally means "without protection". Food proteins can also cause an immediate type of hypersensitivity reaction. The IgE molecule is attached to specific receptors on the surface of mast cells and when activated by linkage to specific allergen inflammatory mediators are released. This is an acute process, hence the name "immediate hypersensitivity".

The initial response occurring within five minutes is due to by the release of histamine, heparin, tryptophan. This is followed by inflammatory mediators—released in five to 30 minutes— leukotrien, prostaglandin. The later response, occurring after some hours, is caused by cytokines—predominantly tumour necrosis factor a (TNF-a) and interleukin 4 (IL4).

Severe reactions cause shock that is made worse by stress and exercise, as in the case of a young woman, allergic to wasp stings, who had a wasp sting when picnicking by a lake. She then plunged into the cold water, swimming vigorously, leading to a fatal anaphylactic reaction. Acute anaphylactic reactions to peanuts may be life threatening.

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