The same immune reaction which protects us from infection can also inflict a great damage not only to the pathogen but also our own body cells and tissues. The immune response uses multiple strategies to reduces the damage. If there is any break down of checks and balances of immune response then it is leading to immune-mediated reactions which are more detrimental than protective. This type of immune-mediated disorder is caused by the failure of immune tolerance. And some other immune-mediated disorder is caused by an inappropriately vigorous innate or adaptive response to the antigen that pose little or no threat. These types of immune-mediated disorders are known as hypersensitivity-reaction.
DEFINITION: Hypersensitivity or hypersensitivity reaction is an undesirable reaction of our immune system (also the allergy and autoimmunity). For hypersensitivity reactions a pre-immune state is required of the host about the stimulus.
There are multiple types of hypersensitivity reactions. According to the time lag there are two types of hypersensitivity, 1. Immediate hypersensitivity reactions, 2. Delayed-type hypersensitivity reaction.
- Immediate hypersensitivity reaction result in symptoms that manifest themselves within very short time periods after the immune stimulus. This type of reaction is mediated by antigen-antibody reactions.
- Delayed-type hypersensitivity is caused by T-cell reactions and take hours or days to manifest themselves.
Hypersensitivity reaction is also divided into four other classes based on the mechanism of immune response done by various antibody and other immune cells. These are:
- Type I Hypersensitivity: It is an immediate and or anaphylactic reaction which is associated with allergy. These reactions are mediated by IgE antibodies and include many of the most common allergies to respiratory allergens like- pollen and dust mites.
- Type II Hypersensitivity: These reactions occurred by the binding of IgG or IgM antibodies to the surface of the host cells, which are then destroyed by complement or cell-mediated mechanism. It is also called Ab-dependent hypersensitivity.
- Type III Hypersensitivity: In these type, the antigen-antibody complexes deposited on the host cells induces complement fixation and enhance inflammatory reaction. It is mainly mediated by IgG antibody, complement proteins and neutrophils.
- Type IV Hypersensitivity: These type reactions result from inappropriate T-cell activation. It takes at least 3 days to develop its action. For this reason, it is also known as cell-mediated or delayed-type hypersensitivity. Some monocytes and macrophages also participate in this type of hypersensitivity.