A human IgA myeloma protein interacting with staphylococcal alpha-toxin and protein A

High anti-staphylolysin activity was found in the serum from a patient with multiple myeloma. The activity was confined to the monoclonal IgA protein and was dependent on an intact Fc part of the molecule. An Fc-dependent interaction with protein A was also demonstrated. In addition, a part of the monoclonal IgA was found to be complexed with alpha2-macroglobulin. The results imply an unusual Fc-reactivity of the IgA protein.