This thesis is based on 9 papers on the uptake of dietary antigens and on the humoral immune responses to dietary antigens, in healthy subjects and in patients with coeliac disease or atopic eczema. Work in experimental animals have indicated that dietary antigens are taken up in amounts, which are nutritionally insignificant, but may be of immunological importance. Local or systemic antibodies may retard the uptake, but in addition may increase the uptake of unrelated antigens. In humans the uptake of intact dietary antigen, free or in immune complexes, was reported in studies in healthy subjects and in patients with immune deficiency or atopy. We investigated the uptake of dietary antigen in 8 healthy subjects after a test meal, using ELISA methods and high performance liquid chromatography (HPLC) for the fractionation of serum samples. A significant finding was that ovalbumin (OA) was taken up as intact antigen or as a high MW immune complex constituent in all the 8 subjects. IgG was the only isotype demonstrable in the OA-containing aggregates, which were detected in particular in the subjects with high anti-OA antibody levels. The antigen was measurable in serum in up to 48 h after the meal. We studied with the same methods the uptake of OA and beta-lactoglobulin (BLG) in children with coeliac disease on a gluten-free diet and after gluten challenge, and in controls with a normal gut mucosa. Both OA and BLG was mostly present in high MW fractions of the sera, presumably as immune complexes. The levels of antigens in serum did not differ between the children with coeliac disease and the controls. However, in 4/5 coeliac children the uptake of both antigens was increased after gluten challenge, indicating increased antigen uptake in coeliac disease. Increased gut permeability has been suggested as a pathogenic factor in food allergy. At present, there is in this condition no direct evidence for altered uptake of dietary antigens. Dietary antigens have been detected in the mother's milk and may be important for the development of normal immunity to dietary antigens and in particular of cow's milk allergy in the infant. Antibodies to dietary antigens have previously been detected in a major proportion of healthy subjects. In a limited number of sera from normals, analysed by electrophoretic techniques, we detected antibodies to bovine serum albumin and OA predominantly in the IgG class. Interestingly, these antibodies were restricted to the IgG subclasses IgG1, IgG2 and IgG4.(ABSTRACT TRUNCATED AT 400 WORDS)