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Excerpt from Hypersensitivity Reactions, ImmediateSynonyms, Key Words, and Related Terms: type I hypersensitivity reactions, allergic reactions, IgE-mediated reactions, immunoglobulin E-mediated reactions, atopy, immunopathology, immediate hypersensitivity reactions, cytotoxic hypersensitivity reactions, delayed hypersensitivity reactions, anaphylaxis, allergic asthma, urticaria, angioedema, allergic rhinitis, drug reaction, atopic dermatitis, inactivation antibody reactions, activation antibody reactions, cytotoxic antibody reactions, cytolytic antibody reactions, immune-complex reactions, T-cell cytotoxic reactions, granulomatous reactions Please click here to view the full topic text: Hypersensitivity Reactions, ImmediateBackground: The immune system is an integral part of human protection against disease, but the normally protective immune mechanisms can sometimes cause detrimental reactions in the host. Such reactions are known as hypersensitivity reactions, and the study of these is termed immunopathology. The traditional classification for hypersensitivity reactions is that of Gell and Coombs and is currently the most commonly known classification system. It divides the hypersensitivity reactions into the following 4 types:
Some authors believe this classification system may be too general and are now in favor of a more recent classification system proposed by Sell et al. This system divides immunopathologic responses into the following 7 categories:
This system accounts for the fact that multiple components of the immune system can be involved in various types of hypersensitivity reactions. For example, T cells play an important role in the pathophysiology of allergic reactions (see Pathophysiology). In addition, the term immediate hypersensitivity is somewhat of a misnomer because it does not account for the late-phase reaction or for the chronic allergic inflammation that often occurs with these types of reactions. Allergic reactions manifest clinically as anaphylaxis, allergic asthma, urticaria, angioedema, allergic rhinitis, some types of drug reactions, and atopic dermatitis. These reactions tend to be mediated by IgE, which differentiates them from anaphylactoid reactions that involve IgE-independent mast cell and basophil degranulation. Such reactions can be caused by iodinated radiocontrast dye, opiates, or vancomycin and appear similar clinically by resulting in urticaria or anaphylaxis. Patients prone to IgE-mediated allergic reactions are said to be atopic. Atopy is the genetic predisposition to make IgE antibodies in response to allergen exposure. The focus of this article is allergic reactions in general. Although some of the clinical manifestations listed previously are briefly mentioned, refer to the articles on these topics for more detail. For example, see Allergic and Environmental Asthma; Anaphylaxis; Food Allergies; Rhinitis, Allergic; and Urticaria. Pathophysiology: Immediate hypersensitivity reactions are mediated by IgE, but T and B cells play important roles in the development of these antibodies. CD4 cells or helper T (TH) cells have been divided into 2 broad classes based on the cytokines they produce.
TH1 cells produce interferon gamma, interleukin (IL)–2, and tumor necrosis factor ..... |
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