ABSTRACT
An immune response directed against an exogenous or an exogenous antigen can result in an immune reaction that has deleterious effects for the host, broadly designated as a hypersensitivity reaction. This chapter describes the classification of hypersensitivity reactions in four types, depending on the clinical characteristics and their pathogenesis that is triggered by antibodies in types I–III but by activated T cells in type IV. In subsequent sections, the most significant examples of reactions of those four types are described. Hay fever, asthma, and anaphylaxis are examples of type I, immediate hypersensitivity. Autoimmune cytopenias and Goodpasture's syndrome are described as examples of type II hypersensitivity reactions resulting in cell and tissue toxicity. Immune complex–induced hypersensitivity reactions are considered as type III, including the Arthus reaction, serum sickness, and several other systemic diseases. Type IV or delayed hypersensitivity is unique because is triggered by activated T cells, and several examples, ranging from poison ivy hypersensitivity to homograft rejection are described in this chapter.
