113lecture2706 - BIOLOGY 113 MICROBIOLOGY Lecture 27...

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BIOLOGY 113 - MICROBIOLOGY Lecture 27: Immunological Disorders - Hypersensitivities, autoimmunity, immune deficiencies Hypersensitivities are immune responses that lead to host damage - Hypersensitivity responses occur in persons who have been "sensitized" to a particular antigen; that is, their immune system has previously encountered the antigen - In the context of hypersensitivities, the relevant antigen is often referred to as an allergen . - Hypersensitivity responses are classified into four principal types (Tortora et al., Table 19.1) = Types I, II and III involve antibody production and are sometimes called immediate hypersensitivities = Type IV involves cell-mediated immune responses, and is sometimes called delayed-type hypersensitivity Type I hypersensitivities, or anaphylactic reactions are what we usually mean by an "allergic response" - Anaphylaxis occurs when antigen binds to molecules of IgE antibody that are attached to the surface of mast cells or basophils (Tortora et al., Figure 19.1) - When antigen binds to IgE attached to a mast cell or basophil, the cell is triggered to undergo degranulation = Degranulation involves release of intracellular granules that contain chemical mediators = It is the mediators that cause the allergic response in tissues - The mediators released upon degranulation include histamine , prostaglandins and leukotrienes ; collectively, they activate inflammation and smooth muscle contraction - Systemic anaphylaxis , or "anaphylactic shock" can occur with some antigens, including venoms, that are introduced into circulation = In anaphylactic shock, release of mediators leads to dilation of peripheral blood vessels throughout the body, resulting in a potentially fatal drop in blood pressure = Epinephrine can be used to counteract the effect of the mediators - Localized anaphylaxis , in which the anaphylactic response is limeted to tissues of the respiratory tract or gastrointestinal tract, is relatively common = "Hay fever" and food allergies are examples of localized anaphylaxis = Antihistamines
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