An IgA High‐Titer Cold Agglutinin with an Unusual Blood Group Specificity within the Pr Complex 1

Abstract A high titer cold agglutinin was present in the serum of a patient with an IgA monoclonal protein. Cold agglutinin activity was confined to the IgA fraction of the separated proteins. The IgA cold antibody appears to have specificity within the Pr complex in that it reacts with adult I, adult i and cord i red cells equally strongly but none of these cells react when treated with papain and neuraminidase. Weak reactions were obtained with dog and rat cells, but only if the cells were not treated with papain or neuraminidase. Monkey, sheep, mouse, hamster, rabbit and guinea pig cells did not react. This pattern of reactions is different from any so far described and may indicate either a qualitative difference, suggesting a further antigen in the Pr complex, or a quantitive difference, suggesting that the Pr1 antigen is present in a weak form on the cells of some animals such as the rat and dog.