Hypersensitivity

HYPERSENSITIVITY REACTIONS
Hypersensitivity refers to undesirable (damaging, discomfort producing and
sometimes fatal), reactions produced by the normal immune system.
Hypersensitivity reactions can be divided into four types: type I, type II, type III and
type IV, based on the mechanisms involved and time taken for the reaction.
Type I Hypersensitivity:
It is also known as immediate or anaphylactic hypersensitivity. The reaction
may involve skin (urticaria and eczema), eyes (conjunctivitis), nasopharynx
(rhinorrhea, rhinitis), bronchopulmonary tissues (asthma), and gastrointestinal tract
(gastroenteritis). The reaction may cause from inconvenience to death. The reaction
takes 15-30 minutes from the time of exposure to antigen. Sometimes the reaction
may have a delayed onset (10-12 hours). Type I reactions are mediated
by IgE antibodies bound to mast cells and basophils. Bound IgE is crosslinked by
allergen, triggering release of pharmacologically active molecules from mast
cells/basophils. These mast cell mediators act on surrounding tissue to induce
pathological edematous inflammatory reactions.