Hypersensitivity responses typically involve normal adaptive protective mechanisms that, because of increased antigenic exposure and/or heightened immune status, become so amplified that they lead to pathologic changes. Hypersensitivity reactions are classified into several basic mechanisms grouped as types I–V (type V is not discussed in this Section; see BCSC Section 9, Intraocular Inflammation and Uveitis). Most clinically relevant ophthalmic diseases are probably not due exclusively to a single type of hypersensitivity reaction. Nevertheless, a basic understanding of the mechanisms of hypersensitivity, as shown in Figure 6-3, can be useful in explaining the pathogenesis of several immune-mediated disorders of the cornea and ocular surface (Table 6-3). In the following sections, each type of hypersensitivity response is put in the context of common corneal and ocular surface inflammatory pathologies. See BCSC Section 9, Intraocular Inflammation and Uveitis, for a full discussion of basic immunopathogenic mechanisms.