Hypersensitivity (or allergy) literally defines an exagerated response of the immune system to antigenic challenge, harmful to the organism itself. Although the basic phenomenology of most types of hypersensitivity reactions was established at the end of the 19th century and in the early years of the 20th century (Koch's phenomenon, Richet and Portier's anaphylaxis, Arthus' phenomenon and serum sickness) , it was only in 1963 that Patrick Gell and Robin Coombs produced a comprehensive classification of hypersensitivity reactions according to their underlying immune effector mechanism.