This chapter has covers all allergic reactions. 1 well illustrated colour table is included to show different allergic reactions and their characteristic features. Allergy reactions, anaphylactic sensitivity, cell-mediated hypersensitivity, complex mediated hypersensitivity, antibody dependent cytotoxic hypersensitivity, hormone allergy reaction are described in detail. 2 references are given at the end of the chapter for further reading. References are mentioned in text at appropriate places for the benefit of readers.