ABO blood group typing is determined by the presence or absence of A or B red blood cell (RBC) surface antigens:
Type A blood has A RBC antigen,
Type B blood has B RBC antigen,
Type AB blood has both A and B RBC antigens,
Type O blood has neither A nor B RBC antigen present.
For emergency transfusions, blood group type O negative blood is the variety of blood that has the lowest risk of causing serious reactions for most people who receive it. Because of this, it's sometimes called the universal blood donor type.
Blood is also classified by rhesus (Rh) factor. If your blood has the Rh factor, you're Rh positive. If your blood lacks the Rh factor, you're Rh negative.
Ideally, blood transfusions are done with donated blood that's an exact match for type and Rh factor. Even then, small samples of the recipient's and donor's blood are mixed to check compatibility in a process known as crossmatching.
In an emergency, type O negative red blood cells may be given to anyone —
especially if the situation is life-threatening or
the matching blood type is in short supply.
Almost all individuals not having A or B antigen “naturally” produce antibodies, mainly immunoglobulin (Ig) M, against those missing antigens within the first year of life.
The patient’s red cells are tested with serum known to have antibodies against A and against B to determine blood type. Because of the almost universal prevalence of natural ABO antibodies, confirmation of blood type is then made by testing the patient’s serum against red cells with a known antigen type.
The patient’s red cells are also tested with anti-D antibodies to determine Rh status. If the subject is Rh-negative, the presence of anti-D antibody is checked by mixing the patient’s serum against Rh-positive red cells. The probability of developing anti-D antibodies after a single exposure to the Rh antigen is 50-70%.
The purpose of this test is to detect in the serum the presence of the antibodies that are most commonly associated with non-ABO hemolytic reactions. The test (also known as the indirect Coombs test) requires 45 min and involves mixing the patient’s serum with red cells of known antigenic composition; if specific antibodies are present, they will coat the red cell membrane, and subsequent addition of an antiglobulin antibody results in red cell agglutination. Antibody screens are routinely done on all donor blood and are frequently done for a potential recipient instead of a crossmatch (below).
A crossmatch mimics the transfusion: donor red cells are mixed with recipient serum. Crossmatching serves three functions: (1) it confirms ABO and Rh typing, (2) it detects antibodies to the other blood group systems, and (3) it detects antibodies in low titers or those that do not agglutinate easily.
In the situation of negative antibody screen without crossmatch, the incidence of serious hemolytic reaction with ABO- and Rh-compatible transfusion is less than 1:10,000. Crossmatching, however, assures optimal safety and detects the presence of less common antibodies not usually tested for in a screen. Because of the expense and time involved (45 min), crossmatches are often now performed before the need to transfuse only when the patient’s antibody screen is positive, when the probability of transfusion is high, or when the patient is considered at risk for alloimmunization.