Interference in immunoassays by human IgM with specificity for the carbohydrate moiety of animal proteins

In allergen characterization common immunoassays such as ELISA and immunoblotting are often used to evaluate human IgE binding of sera from allergic individuals. In an attempt to find and characterize new cat allergens, heavily stained bands were found at 67 and 90 kDa on blotted cat dander extract. Further characterization confirmed these bands to be cat IgA and IgM heavy chains. Analyses in ELISA of purified cat immunoglobulins linked the human IgE binding to cat IgA and IgM and also revealed complete mutual cross-reactivity between the two isotypes. IgE reactivity was found to be directed to carbohydrates of the immunoglobulins and to IgM from 7 out of 9 animal species, but not to human immunoglobulins. Further investigations revealed that the reaction was not mediated by human IgE but by a factor in the serum cross-linking animal immunoglobulins and alkaline phosphatase. The factor was further studied and shown to be human IgM. This IgM fits into the heterophile antibody classification and is directed against carbohydrates on animal immunoglobulins and on calf intestine alkaline phosphatase.