ALLERGIC HYPERSENSITIVITY
In 1964,
Gell and Coombs classified four types of immunologically mediated
hyper-sensitivity states. The majority of disease states encountered in the
clinical practice of allergy are related to type I, or immediate-type hypersensitivity. In this model, an allergen interacts with preformed IgE
on the surface of a mast cell or basophil. This interaction causes
cross-linking of the FceRI receptor and release of multiple mediators,
including histamine, leukotri-enes, and various interleukins. Depending on the
relative localization of release, clini-cal states such as allergic asthma, allergic
rhinitis, or systemic anaphylaxis occur. Of
the remaining clinical allergy hypersensi-tivity states, type IV, or delayed-type hyper-sensitivity, is most
common. In this type, T-cell antigen
receptors on TH1 or TH2 lymphocytes bind to tissue
antigens, caus-ing clonal expansion of lymphocytes and release of
pro-inflammatory lymphokines. Distinct clinical entities such as contact der-matitis (e.g., poison ivy)
or tuberculin skin test sensitivity
in pulmonary tuberculosis may occur relative to the site of the tissue antigen.
Table 9.1 Gell and Coombs Classification of Immune-mediated Allergic Responses
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