Allergy is powerful immune response to an antigen (allergen). The allergen itself is usually harmless (eg. house dust,animal dander,grass pollen). It is therefore usually the immune response that causes damage to the body,not the allergen itself. Upon initial exposure to the allergen the individual becomes sensitised to it,and on second and subsequent exposures the immune system mounts a response entirely out of proportion to the perceived threat. Sometimes symptoms are mild,if annoying,eg the running nose and streaming eyes of hay fever.
MECHANISMS OF HYPERSENSITIVITY
- Type 1,anaphylactic hypersensitivity.This occurs in individuals who have inherited very high levels of immunoglobulin. When exposed to the allergen these levels of antibody activate mast cells and basophils,which release their granular contents. The most important substance released is histamine,which constricts some smooth muscle,causes vasodilation and increases vascular permiability.
- Type II,cytotoxic hypersensitivity. When the antibody reacts with an antigen on a cell surface,that cell is marked for destruction by a number of mechanisms,eg.phagocytosis,or destruction by lytic enzymes.
- Type II,immune-complex-mediated hypersensitivity. Antibody-antigen complexes are usually cleared efficiently when there is phagocyte failure or excessive production of immune complexes,they can be deposited in tissues,eg kidneys,skin,joints and the eye,where they set up an inflammatory reaction.
- Type IV,delayed type hypersensitivity. This involves an overreaction of T-lymphocytes to the antigen. When the antigen is detected,it provokes clonal expansion of the T-lymphocytes,and large numbers of cytotoxic are released to eliminate the antigen.
