Blood Typing
Alternate Names : ABO Blood Typing, Cross Matching, Rh Typing
Definition
Blood typing is a test to classify blood by determining the absence or presence of antigens on the red blood cells and the presence or absence of antibodies to these antigens in the serum.
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Why is the Test Performed?
While the general physical makeup of all people is the same, each individual is unique. Every person has identifiers on cells that allow the body to know that those cells belong to him or her. A and B are common and important identifiers. The O blood type designates the absence of the A and B markers. Another surface identifier, or antigen, on red blood cells is the Rh factor. Whether this antigen is present identifies your blood as Rh+ (positive) or Rh- (negative).
The ABO typing process has two steps: forward and reverse typing. Initially, your blood is mixed with anti-A serum (serum that contains antibodies against type A blood), then with anti-B serum (serum that contains antibodies against type B blood). A determination of the blood type is based on whether or not the blood cells stick together (agglutinate) in the presence of these serums. The blood cells can stick together only when the anti-A antibody binds to the A antigen or the anti-B antibody binds to the B antigen. A laboratory technician can see the cells sticking together when the blood and serum are mixed in a test tube.
The second step involves placing your serum (the liquid portion of the blood without the cells) with blood that is known to be type A and type B (AB). With the results of these two steps, your blood type can be determined accurately.
Rh typing is similar to ABO typing. Here, your blood is mixed with serum containing anti-Rh antibodies and then observed for agglutination. If this occurs, you have Rh-positive blood. If the blood cells do not stick together, you have Rh-negative blood.
Blood typing is especially important during pregnancy. If the mother is found to have Rh-negative blood, then the father should also be tested. If the father has Rh-positive blood, then the mother needs to receive a treatment to help prevent antibodies from forming in the mother's serum. These antibodies can harm a fetus if not treated (see Rh incompatibility).
Blood typing is also necessary before receiving any blood transfusion.
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