A new cold autoagglutinin specificity: the third external loop of band 3

Cold autoagglutinins (CAs) are almost always of immunoglobulin (Ig)M class and specific for carbohydrate antigens. We report a case of hemolytic anemia caused by a mixture of IgM and IgG cold agglutinins. Both agglutinins were specific for the third extracellular loop of band 3.

The patient, a 46-year-old man with no medical history, was admitted for an acute, life-threatening hemolytic anemia caused by a cold agglutinin. Improvement was obtained after plasmapheresis, red blood cell (RBC) transfusions, and steroid therapy. Recovery was complete and no etiology was found.

Conventional serologic procedures and immunochemical methods were used.

The patient's autoantibody was directed to an antigen equally expressed by adult and cord RBCs, resistant to papain, neuraminidase, and endo-beta-galactosidase treatments. This pattern of reactivity excluded all known specificities of cold agglutinins. The antibody failed to react with RBCs treated by alpha-chymotrypsin or pronase, suggesting that it may be directed to the third extracellular loop of band 3. Serum fractionation showed that the cold agglutinin was composed of IgM and IgG class antibodies. Antibody specificity was confirmed by blocking tests using murine monoclonal antibodies against band 3 and by immunoprecipitation.

This is to our knowledge the first observation of a CA specific for band 3. The coexistence of IgM and IgG molecules, a very unusual feature for a CA, may be related to the peptidic nature of the target antigen.