Once the newborn is delivered, exchange transfusion and supportive therapy are begun. Transfusions were not practicable until the four main hereditary blood types, A, B, AB, and O, were discovered at the beginning of this century. There are certain antigens on the surface of red blood cells which can precipitate a transfusion reaction when incompatible blood types are mixed. In the ABO system the types are dictated by A antigen and B antigen. There is also an allele O that does not code for either A or B antigen.
All individuals produce antibodies against the A or B antigens that are lacking on their own cells.
Rh negative individuals lack D antigen on red cells. Another system of blood typing is sometimes considered prior to transfusion of granulocytes or for long-term platelet administration. The typing identifies the HLA antigens that occur on leukocytes and platelets. Crossmatching is another way in which blood is tested for compatibility prior to transfusion. It involves placing the cells of the donor in a sample of the recipient's serum, and cells of the recipient in a sample of the donor's serum.