Serum specimens from apparently healthy adults selected from those who underwent a medical examination at Ajou University Hospital (Suwon, Korea) and from donor RBCs collected from the Korean Red Cross Blood Center were used. These samples were collected from September to December 2012. Aliquots of pooled sera from at least three samples of the same blood group were stored at -70℃ until the ABO Ab titer was measured. Two hundred seventy-four samples were used, and the median age of individuals was 46 yr with a range from 27 to 74 yr