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
Rh blood group system factor is not identical to the rhesus monkey factor; but by then, "Rhesus Group" and like terms were already in widespread, worldwide use. Thus, notwithstanding it is a misnomer, the term survives (e.g. "rhesus blood group system", and the obsolete terms "rhesus factor", " rhesus positive", and "rhesus negative" - all three which actually refer specifically and "only" to the Rh D factor; and, thus, are misleading when unmodified). Contemporary practice is to use "Rh" as a term of art instead of "Rhesus"; viz. "Rh Group," "Rh factors," "Rh D," etc. The first rhesus blood type was discovered in 1937