Immunoglobulin class of antibodies to cow's milk casein in infant sera and evidence for low molecular weight IgM antibodies.

Use of the red cell linked antigen—antiglobulin reaction with specific antiglobulin sera allows the recognition of the immunoglobulin class of the reacting antibody. In the youngest infants IgG was the only immunoglobulin reacting with casein. With the particular sera studied the antibody measured was probably maternal. In older infants IgA and IgM antibodies were also detected but they were usually present at lower levels than IgG antibodies. In serum from a boy with Aldrich's syndrome the IgA level was very high and IgA antibody to casein (titre 128,000) was higher than the IgG antibody. In selected sera containing IgM antibody to casein, evidence is produced of a low molecular weight form of μ-specific antibody. In Sephadex G-200 filtration it was located in the `7S' fraction containing IgG antibody.