Hypersensitivity reactions need a pre-sensitized (immune) state of the host.
Hypersensitivity reactions can be categorized into four types: type I, type II, type III and type IV,
Hypersensitivity Type I
It is also referred as anaphylactic or immediate hypersensitivity.
The reaction may comprise skin (urticaria), nasopharynx (rhinorrhea, rhinitis), eyes (conjunctivitis) and gastrointestinal tract (gastroenteritis) and bronchopulmonary tissues (asthma).
The reaction may origin from minor problem to death.
This reaction usually last for 15-30minutes from the time of experience to the antigen.
Instant hypersensitivity is intermediated by IgE.
The key cellular constituent in this hypersensitivity is mast cell or basophil.
The mechanism of reaction includes special fabrication of IgE in response to some antigens and allergens.
IgE has very high affinity towards receptor on mast cells as well as basophils.
A succeeding introduction to the same allergen cross links the cell-bound IgE and causes the release of numerous pharmacologically active substances.
They cause the hypersensitivity reactions.
Hypersensitivity Type II
It is considered as cytotoxic hypersensitivity and can harm several organs and tissues.
For example, Drug-Induced hemolytic anemia, granulocytopenia and thrombocytopenia.
Hypersensitivity Type III
It is also called as immune complex hypersensitivity. The reaction may be common like serum sickness or may include specific organs comprising skin , kidneys (eg. lupus nephritis), lungs (eg. aspergillosis)etc.
Hypersensitivity Type IV
It is commonly referred as delayed type or cell mediated hypersensitivity.
The typical example of this hypersensitivity is tuberculin (Montoux) reaction which mounts 48 hours later the injection of antigen (old tuberculin or PPD).
Type IV hypersensitivity plays significant role in the pathogenesis of numerous autoimmune and infectious diseases like (blastomycosis, leprosy, tuberculosis, histoplasmosis,many more).
