Hypersensitivity refers to a condition in which immune response results in excessive reaction leading to tissue damage, disease or even death in sensitised host.
Hypersensitivity reactions are immune responses that are exaggerated for inappropriate against an antigen or allergen.
- This reaction need previous contact with antigen (allergen).
- Initial contact sensitise immune system by priming appropriate Tor B lymphocyte, this is known as priming or sensitising dose.
- Subsequent contact with same antigen cause hypersensitivity, this is known as shocking dose.
Hypersensitivity reactions are of two types-
Immediate and delayed
1. IMMEDIATE TYPE
- Appears and recedes slowly.
- Antibody mediated immune response.
- Passive transfer possible with serum.
- Desensitisation easy but short lived.
- Induction by antigens or haptens (by any route)
2. DELAYED TYPE
- Appears slowly in 24-72 hours and lasts longer.
- Cell mediated immune response.
- Passive transfer is possible with lymphocyte or transfer factor (not by serum)
- Desensitisation is difficult but long lasting.
- Induction by antigen injected intradermally or by skin contact.
Coomb and Gel (1963) classified hypersensitivity into four major types
TYPE 1- Anaphylactic
TYPE 2- Cytotoxic
TYPE 3- Immune complex
TYPE 4- Delayed or cell mediated
A.TYPE 1 (ANAPHYLACTIC)
Anaphylactic response is mediated by IgE antibodies that are produced by immune system in
response to environmental proteins
Such as pollens, animal dander or dust mites. These antibodies bind to mast cells and
basophils which contain histamine granules that are released in reaction and cause inflammation.
It can be seen in bronchial asthma, allergic rhinitis, allergic dermatitis, food allergy, allergic conjunctivitis, anaphylactic shock.
B.TYPE 2 (CYTOTOXIC)
IgG or IgM mediate cytotoxic mediated response against cell surface and extracellular matrix
protein. The immunoglobulins involved in this type damages cells by activating complement
system or by phagocytosis.
Type 2 hypersensitivity reactions can be seen in Immune thrombocytopenia, autoimmune
haemolytic anaemia, and autoimmune neutropenia.
C.TYPE 3 (IMMUNE COMPLEX)
These are also mediated by IgM and IgG antibodies that react with soluble antigens forming
antigen antibody complexes.
The complement system become activated and releases chemotactic agents that attract
neutrophils and cause inflammation and tissue damage as seen in vasculitis and
Type 3 hypersensitivity reactions can classically be seen in serum sickness and Arthus
D. TYPE 4 (DELAYED TYPE)
This reaction is mediated by T lymphocytes.
T lymphocytes on contact with specific antigen release lymphokines that cause biological
effect on macrophages, leucocytes abd tissue cells.It occurs in 48-72 hours.
Two types of type 4 hypersensitivity reaction are.
1 .Tuberculin type
2 contact dermatitis type