Blood groups are complex. The two best known blood group systems are ABO and Rh. Within each blood group, red cells can carry surface markers called antigens. For example, within the ABO blood group system, there are the A and B antigens — people with A have the A antigen, people with B have the B antigen, people with AB blood have both and people with O have neither. There are now 47 recognized blood group systems together containing more than...
