Hypersensitivity reactions

Hypersensitivity reactions are abnormal, harmful immune responses that are intended to protect the body but instead result in disease and damage to the body's own tissues. They are classified into four types: Type I, II, III and IV, based on the mechanisms involved and time taken for the reaction. Frequently, some diseases are associated with specific types of hypersensitivity reactions.

Type I Hypersensitivity

Type I hypersensitivity is also known as an immediate reaction and occurs within minutes of exposure to an known antigen. The reaction may be localized, as in allergic asthma or systemic (anaphylaxis), where an often severe response including hypotension, respiratory distress and even death can occur.

Type II Hypersensitivity

Type II hypersensitivity is also known as cytotoxic hypersensitivity and may affect a variety of organs and tissues. The antigens are normally endogenous, although they may also be exogenous chemicals, drugs or infections bound to cell surfaces. Examples include Goodpasture's syndrome, pemphigus and autoimmune hemolytic anemia.

Type III Hypersensitivity

Type III hypersensitivity occurs when there is an excess of antigen, leading to immune complexes being formed in the blood and deposited in various tissues, where they may trigger an inflammatory response. This can result in vasculitis, glomerulonephritis, lupus erythematosus and other conditions.

Type IV Hypersensitivity

Type IV hypersensitivity is also known as cell mediated or delayed type hypersensitivity. This is the only type of hypersensitivity that is not antibody mediated but rather is a type of cell mediated response. This type of reaction is seen in many autoimmune and infectious diseases (tuberculosis, leprosy, blastomycosis, histoplasmosis, toxoplasmosis, leishmaniasis etc.) as well as in contact dermatitis and transplant rejection.

See Also

• Allergy
• Immunology
• Autoimmune diseases
• Transplant rejection

References

Hypersensitivity reactions Resources
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