ABO blood group system A-type blood, with A2 making up almost all of the rest. These two subgroups are not always interchangeable as far as transfusion is concerned, as some A2 individuals produce antibodies against the A1 antigen. Complications can sometimes arise in rare cases when typing the blood. With the development of DNA sequencing, it has been possible to identify a much larger number of alleles at the ABO locus, each of which can be categorized as A, B, or O in terms of the reaction to transfusion, but which can be distinguished by variations in the DNA sequence. There are six common
Blood type individuals with hh antigen system (also known as the Bombay phenotype) who can only receive blood safely from other hh donors, because they form antibodies against the H antigen present on all red blood cells. Blood donors with exceptionally strong anti-A, anti-B or any atypical blood group antibody may be excluded from blood donation. In general, while the plasma fraction of a blood transfusion may carry donor antibodies not found in the recipient, a significant reaction is unlikely because of dilution. Additionally, red blood cell surface antigens other than A, B and Rh D, might cause adverse reactions and sensitization,