dhypersensibilité

Hypersensitivity refers to an exaggerated or inappropriate immune response to an antigen that would be tolerated by most other individuals. This reaction is mediated by the immune system, leading to tissue damage or inflammation. While the immune system's primary role is to protect the body from pathogens, in hypersensitivity reactions, it mistakenly attacks harmless substances or the body's own tissues.

There are four main types of hypersensitivity reactions, classified by Gell and Coombs. Type I, IgE-mediated hypersensitivity, involves the rapid release of mediators like histamine when an allergen binds to IgE antibodies on mast cells. Examples include allergic asthma and anaphylaxis. Type II, cytotoxic hypersensitivity, involves antibodies (IgG or IgM) binding to cell-surface antigens, leading to cell destruction. This can occur in transfusion reactions or autoimmune hemolytic anemia. Type III, immune complex-mediated hypersensitivity, results from the deposition of antigen-antibody complexes in tissues, triggering an inflammatory response. Examples include serum sickness and lupus nephritis. Type IV, delayed-type hypersensitivity, is a cell-mediated response involving T cells, typically occurring 24-72 hours after exposure to an antigen. This is seen in contact dermatitis and the tuberculin test. Understanding these mechanisms is crucial for diagnosing and managing allergic and autoimmune conditions.