Clinical Significance of Anti‐At^a

At^a is a high‐frequency red blood cell (RBC) antigen. Anti‐At^a has been reported in rare At(a–) black subjects. We report two cases of anti‐At^a. A clinically significant anti‐At^a was found in a 26‐year‐old black woman with systemic lupus erythematosus. The patient had a transfusion reaction with chills and nausea during a RBC survival study, and 95% of the radiolabeled At(a+) RBCs were destroyed within 3 h. A concurrently performed monocyte monolayer assay was strongly reactive. Anti‐At^a thus can cause rapid hemolysis of transfused RBCs, but At(a–) donor units are extremely scarce in rare donor registries. A second patient at our hospital had anti‐At^a which did not affect her newborn. She also had autoimmune disease, insulin‐dependent diabetes mellitus.