Type I Hypersensitivity (atopic allergy)

Type I hypersensitivity is characterized by an allergic reaction that occurs immediately following contact with the antigen, referred to as the allergen. Atopic allergy is mediated by immunoglobulin E (IgE), antibodies which are bound to the mast cells. When allergens cross-link the IgE molecules degranulation is induced and mediators such as histamine and other vasoactive amines are released and produce the allergic reactions. Circulating IgE antibodies can be measured in serum (total and allergen specific) and IgE levels are often raised in allergic individuals. Common environmental allergens inducing IgE-mediated allergy are cat-, dog- and horse epithelium, tree-, grass-, and weed pollen, house dust mites and molds. Food allergens are also a common cause of allergy, even though food allergy is more frequently seen in children than in adults.

 

The term atopy was coined in 1923 by Coca and Cooke to describe clinical allergy of an inherited nature. Nowadays an atopic constitution implies a hereditary tendency to produce immunoglobulin E to common environmental allergens and an increased risk of developing asthma, rhino-conjunctivitis (hay fever), urticaria and atopic eczema. Atopic patients also have an increased tendency to be allergic to food substances, which can result in various symptoms, including those described above plus others of the gastrointestinal tract.

 

IgE plays a major beneficial role in the defense against parasitic worms, allergies being an unfortunate by-product.

 

 
Reference: Roitt I, Brostoff J, Male D. Immunology.
5th edition. Hypersensitivity – Type I. London:
Mosby, 1998:301 (1).