The role of IgG in type-I allergy: an unsolved problem

The role of antiallergen IgG antibodies in allergy remains unclear. In this review we present evidence for and against the hypothesis that IgG (IgG4) could act as a sensitizing antibody. After considering the available data, we conclude that the possible sensitizing ability of IgG4 may depend on the nature and origin of the antibodies and cells used in the various experiments and/or is related to the differential induction of two subtypes of IgG4 (anaphylactic vs. blocking). It still remains to be settled whether the observed increase in specific IgG during immunotherapy is causally related to the relief of symptoms, or whether it merely represents an epiphenomenon due to high antigen exposure. Furthermore, the presence of IgG anti-IgE antibodies in the serum of allergic patients and their possible pathophysiological relevance is discussed in the light of recent evidence suggesting that the raised levels of these antibodies may likewise represent an epiphenomenon of the immune response. There exists a tight relationship between antiallergen IgE and IgG antibodies. This correlation has led to the suggestion that IgG determinations might be useful for diagnostic purposes. There also exists a good correlation between these antiallergen isotypes in inhibition assays. It has therefore been proposed to employ inhibition of IgG binding for the standardization of allergenic extracts. More recent studies explore the relevance of the binding affinities or avidities of allergen-specific IgG antibodies. Antibody affinity may have important repercussions with respect to the biological effects. It has been pointed out that the affinity of specific IgG subclasses in allergic patients depends, among other things, on the nature of the sensitizing allergen. Further studies on IgG binding to the so-called "major" and "minor" allergens may help clarify the role of IgG in allergic disorders.