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Type I hypersensitivity (or immediate hypersensitivity) is an allergic reaction provoked by reexposure to a specific type of antigen referred to as an allergen. Type I is not to be confused with Type II, Type III, or Type IV hypersensitivities. Exposure may be by ingestion, inhalation, injection, or direct contact. ==Pathophysiology== In type 1 hypersensitivity, B-cells are stimulated (by CD4+TH2 cells) to produce IgE antibodies specific to an antigen. The difference between a normal infectious immune response and a type 1 hypersensitivity response is that in type 1 hypersensitivity the antibody is IgE instead of IgA, IgG, or IgM. During sensitisation, the IgE antibodies bind to Fcε receptors on the surface of tissue mast cells and blood basophils.〔(【引用サイトリンク】title=The Adaptive Immune System: Type I Immediate Hypersensitivity )〕 Mast cells and basophils coated by IgE antibodies are "sensitised." Later exposure to the same allergen cross-links the bound IgE on sensitised cells, resulting in degranulation and the secretion of pharmacologically active mediators such as histamine, leukotriene (LTC4 and LTD4), and prostaglandin that act on the surrounding tissues. The principal effects of these products are vasodilation and smooth-muscle contraction. Type 1 hypersensitivity can be further classified into an immediate and late-phase reaction. The immediate hypersensitivity reaction occurs minutes after exposure and includes release of vasoactive amines and lipid mediators, whereas the late-phase reaction occurs 2–4 hours after exposure and includes the release of cytokines. The reaction may be either local or systemic. Symptoms vary from mild irritation to sudden death from anaphylactic shock. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Type I hypersensitivity」の詳細全文を読む スポンサード リンク
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