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hypersensitivity n.
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Hypersensitivity

Hypersensitivity

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Hypersensitivity

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  1. Hypersensitivity An inappropriate or exaggerated immune response to an antigen... which is either exogenous (present in the environment) or endogenous (within the body)

  2. Hypersensitivity • Hypersensitivity reactions: occur when the immune system recognise foreign antigens, but cause other tissue damage whilst destroying the intended antigen • Atopy: is a genetic predisposition to allergy characterised by IgE production to common antigens (eg. pollen)

  3. Classification • Type I – immediate hypersensitivity, or allergy, due to activation of IgE antibody on mast cells or basophils • Type II – antibody to cell-bound antigen • Type III – immune complex reactions • Type IV – delayed hypersensitivity mediated by T-cells • The first three are antibody mediated, the fourth is caused by T-lymphocytes (cell mediated)

  4. Type I - Stages

  5. Antibody Mediated – Type I • Local Atopic Allergic • Once sensitised, exposure to antigen causes a fast, symptomatic response – with the same signs as in inflammation (which are?) • Examples: allergic rhinitis (hay fever), asthma (two types, extrinsic – accompanying other allergies and intrinsic – stress, cold air, exertion) & eczema • General Allergic; Anaphylaxis • Systemic reaction, life-threatening • Caused by generalised degranulation of IgE-sensitised mast cells and basophils • Obstruction of airways, mm spasm, increase BP

  6. Antibody Mediated – Type II • Triggered by antibodies reacting with antigenic determinants which form part of the cell membrane • IgM and IgG antibodies typically implicated • Cell destruction is brought about by recruiting complement • Examples: • Transfusion reaction (IgM), incorrect blood gp causes agglutination • Haemolytic disease (IgG), rhesus factor causes lysis of RBC

  7. (Antibody Mediated – Type V) • Antibodies recognise and bind to the cell surface receptors, which either presents the intended ligand binding with the receptor, or mimics the effect of the ligand thus impairing cell signalling • It is distinguished from Type II because the antibody is stimulating cell function • Example: Graves disease, where auto-antibodies against the TSH receptor drive over-production of hormones by the cell

  8. Antibody Mediated – Type III • Result from the deposition or formation of immune complexes in the tissues • If these complexes accumulate in lg quantities, they may activate complement and accessory cells and produce extensive tissue damage • Characterised by IgM • Examples: • Arthus reaction: antigen is injected into the skin of an animal that has been previously sensitised; inflammation 6-24 hrs after • Systemic Lupus Erythematosus (SLE): antigen-antibody complexes are deposited in the kidney, causing glomerulonephritis. Affects other organs (watch this space)

  9. Cell Mediated – Type IV • Involves T lymphocytes that attract macrophages and cytotoxic T cells • DTH – Delayed-type Hypersensitivity reaction: a local inflammatory response taking 2/3 days to develop • Examples: • Mantoux / Heaf Test: prick test for BCG • Graft rejection: immunosuppressive drugs needed

  10. Bedtime reading • http://pathmicro.med.sc.edu/ghaffar/hyper00.htm