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