ABSTRACT
The immunological basis for allergy has been discussed in Chapter 12. Allergy is the pathological manifestation of hypersensitivity and may be defined as ‘a state of immunological sensitization to an innocuous environmental antigen that leads to an excessive (symptomatic) immune response on reexposure to the antigen’. Although allergy may theoretically involve any of the four hypersensitivity mechanisms described in Chapter 12, most clinically significant allergic diseases are manifestations of type I or IV hypersensitivity. The word ‘atopy’ is used to describe the genetically-mediated propensity of an individual to become sensitized to environmental allergens by producing allergen-specific IgE antibodies that mediate immediate hypersensitivity reactions. Such individuals are referred to as being ‘atopic’ and probably have overactive Th2 immunity as a consequence of impairment of natural Treg cell function (237).
