IMMUNOLOGY - Roit A. - Mir 2000

Chapter 23. Hypersensitivity - Type I

THE BENEFICIAL ROLE OF IgE

Given that such numerous adverse effects are associated with the IgE response, the question arises as to why Antibodies of this Class ever evolved in the first place.

It has long been believed that IgE plays a primary role in defense against helminths; the mechanisms of this effect are illustrated in Fig. 18.9. Helminths parasitizing the intestine secrete soluble allergens that provoke an exceptionally strong IgE, as well as IgG, response in the gut-associated lymphoid tissue (GALT). Mast Cells maturing in the GALT are sensitized by IgE and migrate into the intestinal mucosa, where, upon contact with helminth Antigens, these cells undergo degranulation, releasing mediators that increase vascular permeability and recruit inflammatory cells, particularly eosinophils, to the area. IgE from the Lymph Nodes also sensitizes the helminths themselves to the action of eosinophils, which possess Fc receptors for IgE. As a result of the mediator-induced increase in vascular permeability—caused by substances such as histamine—Complement and helminth antigen-specific IgG also enter the inflammatory site. Simultaneously, mucus production by mucosal goblet cells increases. All of these factors lead to the damage and expulsion of the helminths from the body.

Since approximately one-third of the global human population is infected with helminths, it is quite possible that the IgE class arose as a result of selective pressure, with allergy thereby turning out to be an undesirable evolutionary side effect.

Questions for Structure/133.html">Discussion

■ Are Th2 cells (and the cytokines they produce) the single most important factors in the Pathogenesis of allergy? Is this statement correct?

■ If you were engaged in developing new therapeutic approaches for treating allergies, which path would you follow?

■ What measures should be taken for the specific blockade of late-phase reactions in the Lungs, and will this affect the asthmatic response?

■ How can the relative roles of genetic and environmental factors in The Development of allergy be investigated?

■ What accounts for the spread of allergic diseases among the population—from 1–2% in the 1900s to nearly 30% at present?

■ Is allergic asthma truly an example of delayed-type hypersensitivity in the lungs?

Introduction/47.html">Further Reading

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