Immune response recruits and mobilizes a series of effector molecules that induce a localized inflammatory response, which ultimately removes the antigen.
This inflammatory response usually does not extensively damage the host tissues. But under certain conditions, the inflammatory response produces deleterious effects, resulting in significant tissue damage or even death; this is termed as hypersensitivity or allergy.
Hypersensitive reactions may develop in the course of either humoral or cell-mediated immune responses. The phenomenon of hypersensitivity was discovered during the early 19th century by two French scientists. Porter and Richet, who concluded that toxin from a kind of jelly fish induced ‘overreaction’ in dogs; they termed this overreaction as anaphylaxis, which is a type (Type III) of hypersensitivity.
Hypersensitivity has been classified into the following four types by Gell and Coombs:
(i) IgE-mediated (Type I) hypersensitivity,
(ii) Antibody-mediated cytotoxic (Type II) hypersensitivity,
(iii) Immune-complex- mediated (Type III) hypersensitivity, and
(iv) Delayed-type (Type IV) hypersensitivity (DTH).
The first three types of hypersensitivities are grouped together as immediate hypersensitivity since the symptoms become manifest within minutes or hours after a sensitized recipient encounters the antigen. The DTH, on the other hand, is so called because symptoms develop days after the exposure to the antigen. The features of the four types of hypersensitive responses are summarised in Table 43.1.