The ABO system is the first system considered when it comes to transfusions. A Type O individual can only receive Type O blood, while a Type AB individual can receive A, B, AB, and O. This system was first described by Karl Landsteiner in 1900, which was a landmark achievement in the world of blood banking. The ABO antigens are found on red cells and platelets, as well as many circulating proteins and even various tissues in the body (which comes into play for organ transplantation). Failure to properly match ABO types for blood transfusions can result in acute intravascular hemolysis, renal failure, and death; in organ transplants, it is possible to see acute humoral rejection of the transplanted organ. As such, ABO testing is of critical importance in any pretransfusion scenario and is of high importance before transplantation.
Apart from the rare Bombay phenotype, all red blood cells express the H antigen. This antigen is expressed by itself on Type O red blood cells and is the precursor for building the A and B antigens. Its expression decreases–depending on the blood type of the individual–in the following sequence: O > A2 > B > A1 > A1B. A1 and A2 are subgroups of the A antigen and will be discussed in a later post. The H antigen is composed of two Galactose (a sugar of the hexose class that is a constituent of lactose and many polysaccharides), an N-acetylglucosamine (a monosaccharide derivative of glucose), and a Fucose (a six-carbon monosaccharide produced in plant polysaccharides).
The genes for the ABO system are located on chromosome 9. While genetics play a part in the expression or non-expression of the A and B antigens, genetics operates in an indirect role. A and B antigen expression is caused when a sugar is added to a polypeptide (forming a glycoprotein) or a lipid (forming a glycolipid) through enzymatic activity via sugar-transferring enzymes collectively called glycosyltransferases. Three other antigen systems have their expressions determined in similar fashion: Lewis, Hh, and P. These, along with the ABO system, are known as carbohydrate-defined antigens. As far as I have seen, all other antigens are determined by amino acid sequences of proteins that are directly determined by genes.
In Type A and AB individuals, the A antigen is expressed. This antigen is created when an enzyme, 1-3-N-acetylgalactosaminyltransferase, attaches an N-acetylgalactosamine (an amino sugar derivative of galactose) to the H antigen. The A gene is very similar to the O gene; the difference is that the O gene has one base deletion that causes a frame shift and premature stop in transcription. Since I am Type O, this shows that I am a genetic miscreant. 😉
The B antigen is created when the enzyme 1-3-galactosyltransferase attaches a galactose to the H antigen. The B antigen is formed in individuals who are either Type B or AB. It is sufficient for our purposes here to say that the B gene is different from the A gene in several ways.
This discussion is highly limited in terms of the actual literature, and I encourage you to do further research if this topic interests you. It is possible that you may find Blood Banking materials through your public library system, but it is more probable that a university library system would have the information you desire. If you can gain access to online journals, Transfusion, published by AABB in Philadelphia, is an excellent source.
 McCullough, Jeffrey, MD. Transfusion Medicine. 2nd ed. Philadelphia: Churchill Livingstone, 2004. Print.
NOTE: None of the information in this post is designed to take the place of any information you might receive through a physician; I am not a doctor. I am a Medical Technologist and am tasked with performing diagnostic testing on blood, urine, and other body fluids, but I cannot be expected to give you medical advice or direction. I simply enjoy what I do and am privileged to share some of what I’ve learned with you. I have done my best to present accurate information in this post, but know that I am only human and subject to error.