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Conjunctivitis - allergic - Background information
What is it?
- Allergic conjunctivitis is inflammation of the conjunctiva due to a hypersensitivity reaction that is triggered by exposure to an allergen. Four common types of allergic conjunctivitis are recognized: seasonal, perennial, giant papillary, and contact dermatoconjunctivitis.
- Two types of hypersensitivity reaction occur in allergic conjunctivitis:
- Type I hypersensitivity reactions occur immediately following contact with the allergen, causing mast cells to degranulate and release histamine. Histamine causes itch, as well as dilatation and increased permeability of conjunctival blood vessels, which results in red eye and oedema. Inflammation of the cornea is rare.
- Type IV hypersensitivity reactions develop 24–48 hours after contact with an allergen and lead to inflammation without degranulation of mast cells or release of histamine. Inflammation of the cornea may occur.
- Table 1 shows the hypersensitivity reactions that occur, their usual causes, and response to treatment, for the four common types of allergic conjunctivitis.
Table 1. Hypersensitivity reactions involved in the common types of allergic conjunctivitis.
Type of allergic conjunctivitis | Type of hypersensitivity reaction | Response to antihistamines and mast cell stabilizers | Usual causes |
|---|
Seasonal conjunctivitis | Pure type I | Responds well | Pollens |
Perennial conjunctivitis | Pure type I | Responds well | House dust mites, animal dander |
Giant papillary conjunctivitis | Mixed type I and type IV | Responds partially | Contact lenses, sutures, and prostheses following eye surgery |
Contact dermatoconjunctivitis | Pure type IV | Does not respond | Eye drops, cosmetics, chemicals |
[Freissler et al, 1997; Yanoff and Duker, 2004]
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