It is common to find anti-food IgG antibodies circulating in blood, even in people who have no signs or history of adverse reactions to foods. In fact, some authorities think that an increase in anti-food IgG in some cases might be indicative of successful resolution of an IgE-mediated allergy. The subject of IgG-mediated food allergy is extremely complicated because of the nature of the antibody and the immunological reactions associated with it. The present knowledge about food allergy-associated IgG may be summarized as follows:
• Four distinct subclasses of IgG have been identified, designated IgG1, IgG2, IgG3, and IgG4.
• IgG4 seems to be the subclass that has a high affinity for food antigens.
• IgG4 represents a very small proportion of total IgG in normal sera; reports of the normal level of IgG4 differ from laboratory to laboratory, the range being 0.7 percent to 4.9 percent of total IgG.
• In the newborn baby, levels of IgG1 and IgG3 rise rapidly. IgG1 may reach concentrations close to the adult level at 8 months: In contrast, IgG4 is still only a fraction of the adult level at 2 years of age and may not reach adult levels until 12 years.
• Anti-food IgG4 may be associated with allergy, in particular to the milk protein ß-lactoglobulin in atopic dermatitis (eczema) in children.
• There is some evidence that total anti-food IgG (all classes of IgG measured together) might represent some protection from IgE-mediated food allergy: In one study, symptom-free children had higher levels of IgG antibodies to milk and egg proteins than those who developed allergic symptoms, and a high IgG/IgE ratio in cord blood was suggested to be a sign of a decreased risk for food allergy.
• Food allergy in infants is frequently associated with an increase in gut permeability (“leaky gut”). It is likely that antigenic food molecules passing into circulation trigger production of anti-food IgG. Thus, in cases of IgE-mediated food allergy that result in inflammatory reactions within the gastrointestinal tract causing a non-intact digestive epithelium, it is logical to expect to find higher than normal levels of anti-food IgG. Some authorities think that these anti-food IgG antibodies represent a protective mechanism, rather than a source of allergic pathology.
It remains for future research to determine the role of IgG, and especially IgG4 anti-food antibodies, in allergy. We shall look at the consequences of all this immunological activity in our discussion of the symptoms of allergy in the baby and child.
But first we should start right at the beginning, and ask the most important question: Is there anything that we can do to recognize the baby who is likely to develop allergy and thereby prevent the development of allergy in the first place?
