Serum IgG antibodies reactive with different dietary proteins have been detected in a significant proportion of adult patients with coeliac disease, dermatitis herpetiformis and atopic eczema. Serum anti-milk antibodies were shown to be distributed predominantly between the IgG2 and IgG4 subclasses, whereas anti-gliadin antibodies in atopic eczema were predominantly of the IgG4 subclass. Furthermore, as antibodies to each of these dietary antigens in healthy adults were markedly restricted to the IgG4 subclass, their production may be part of a normal immune response to dietary proteins. There was no correlation between serum IgG4 antibody and total serum IgG4 level. In contrast, restricted IgG4 anti-gliadin antibodies were less prevalent in the serum of patients with coeliac disease and dermatitis herpetiformis, suggesting defective downstream switching of Ig heavy-chain genes in these conditions.

The isotype distribution of human IgG antibodies reactive with common dietary proteins has been evaluated in sera from adult patients with the irritable bowel syndrome and with bronchial asthma using a solid-phase immunoassay (ELISA). In both these medical disorders, serum antibodies reactive with ovalbumin or gliadin were restricted predominantly to the IgG4 isotype; however, IgG antibodies reactive with bovine milk antigens, notably casein, were often restricted to both the IgG2 and IgG4 isotypes. A similar serum IgG antibody isotype distribution for these dietary protein antigens was also demonstrated in IgG antibody-positive healthy adults. These data amplify the view that production of antibodies of the IgG4 isotype may reflect a normal immune response to dietary protein antigens presented at mucosal surfaces.

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The antigenic specificity of human serum IgG antibodies reactive with common dietary proteins has been evaluated by competitive binding using a solid-phase immunoassay (ELISA). Antibodies reactive with bovine milk antigens were shown to be reactive predominantly with casein, rather than α-lactalbumin, β-lactoglobulin, γ-globulin or albumin. Furthermore, sera containing antibodies reactive with bovine casein, wheat gliadin and chicken ovalbumin showed competitive binding only by each respective dietary protein antigen. IgG4 antibodies specifically reactive with ovalbumin, gliadin or casein were also not cross-reactive in competitive binding studies. Furthermore, both IgG2 and IgG4 anti-milk antibodies showed significant inhibition only with bovine casein, and not with α-lactalbumin or β-lactoglobulin. These data are relevant to concepts regarding the immunobiological role of antibodies of the IgG4 isotype.

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