The immunological basis of immediate hypersensitivity

IgE is a homocytotropic antibody which binds to the surface of the mast cell. Antigen with affinity for IgE triggers conformational change at the cell surface, resulting in the release of chemical mediators from the mast cell granules. The mediators histamine, slow reacting substance of anaphylaxis and eosinophil chemotactic factor cause smooth muscle contraction, increased capillary permeability, eosinophil attraction and increased glandular secretions. The release of mediators from the mast cell granules is controlled by intracellular levels of cyclic nucleotides. In particular, elevated cyclic AMP inhibits mediator release. Adrenergic, cholinergic and prostaglandin receptors all influence mediator release. The characteristic immunopathology of immediate hypersensitivity reactions is a result of local or systemic mediator release. Such reactions include anaphylaxis, asthma, allergic rhinitis, urticaria and angioedema. Similar immunopathology may sometimes result from mechanisms not involving IgE or histamine mediators. Routine investigation of patients with immediate hypersensitivity should include eosinophil counts and IgE levels in blood and secretions, and immediate hypersensitivity skin tests. RAST testing is not routine. Therapeutic principles of these reactions include restoration of inhibitory levels of cyclic nucleotides, antagonism of mediator effects and immunological manipulation of the IgE mediated allergic reaction.