Hypersensitivity and Types of Hypersensitivity reaction
- When immune system works properly, it helps in elimination or removal of antigens from host body by means of effector molecules. The effectors molecules generally induce local inflammatory response and removes antigen without extensively damaging host tissues. However in certain conditions such as when immune system does not work properly or over activated, it causes deleterious effects resulting in significant tissue damage, serious disease and even death. Such unwanted, inappropriate and damaging immune response is termed as hypersensitivity.
- Although the word ‘hypersensitivity’ implies an increased response, the immune response in hypersensitivity reaction is not always heightened but may instead be an inappropriate response.
- Hypersensitivity reaction may develops in course of either cell mediated or humoral immune response
Gell and Coomb’s Classification of Hypersensitivity reaction
- Hypersensitivity reactions are classified as Immediate or delayed type on the basis of time required by sensitized host to response to shocking dose of antigen.
- Hypersensitivity reactions can be distinguished by immune response and difference in effectors molecules generated in course of reactions.
- G.H Gell and R.R.A Coomb classified hypersensitivity reactions into four types.
- Type-I hypersensitivity reaction: IgE antibody mediated
- Type-II hypersensitivity reaction: Antibodies mediated
- Type-III hypersensitivity reaction: Antigen-antibody complex mediated
- Type-IV hypersensitivity reaction: Activated T-cell and cytokines mediated
** Two additional types, Type-V and Type-VI hypersensitivity reaction are also proposed.
5. Type-V hypersensitivity reaction: Antibody mediated
6. Type-VI hypersensitivity reaction: Both antibody and cell mediated
Immediate hypersensitivity reaction:
- Immediate hypersensitivity reaction is mediated by humoral antibody and manifests within minutes to few hours.
- The anaphylactic reaction initiated by antibody or Ag-Ab complex are referred as immediate hypersensitivity because it occurs within minutes or few hours after a sensitized host encounter with antigen.
- Type-I, II and III hypersensitivity reaction are immediate
Delayed hypersensitivity reaction:
- Delayed hypersensitivity reaction is mediated by sensitized CD4 T cells and manifests slowly usually after 24 hours or more.
- It is called delayed because symptoms appears days after exposure to antigen.
- In delayed hypersensitivity reaction, the effector molecules are various cytokines secreted by activated CD4 T cells themselves.
- Type-V hypersensitivity depends upon activation of T cells and occurs within cell mediated branch of immune response and termed as delayed type hypersensitivity reaction (DTH).