Hypersensitivities

Hypersensitivities are abnormal reactions of the immune system that in a normal person would be of no consequence. They are generally divided into four types: Type I, II, III and IV, based on the mechanisms involved and time taken for the reaction. Frequently, some types of hypersensitivity overlap with autoimmune disorders.

Types of Hypersensitivities

Type I Hypersensitivity

Type I Hypersensitivity is also known as immediate or anaphylactic hypersensitivity. The reaction may involve skin (urticaria and eczema), eyes (conjunctivitis), nasopharynx (rhinorrhea, rhinitis), bronchopulmonary tissues (asthma) and gastrointestinal tract (gastroenteritis). The reaction may cause a range of symptoms from minor inconvenience to death. The reaction is caused by the crosslinking of the IgE on mast cells and basophils by the antigen, causing a release of preformed mediators.

Type II Hypersensitivity

In Type II Hypersensitivity reactions, the antibodies produced by the immune response bind to antigens on the patient's own cell surfaces.

Type III Hypersensitivity

Type III Hypersensitivity occurs when there is an excess of antigen, leading to small immune complexes being formed that fix complement and are not cleared from the circulation. It involves soluble antigens that are not bound to the cell surface. Thus, when these antigens bind antibodies, immune complexes (type III hypersensitivity) are formed in the circulation and are later deposited in the vessel walls and other tissues.

Type IV Hypersensitivity

Type IV Hypersensitivity is often called delayed type hypersensitivity as the reaction takes two to three days to develop. Unlike the other types, it is not antibody mediated but rather is a type of cell-mediated response.

See Also

• Allergy
• Autoimmune diseases
• Immunology

References

Hypersensitivities Resources
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