May 23, 2012 - The “positive” or “negative” refers to the Rh (Rhesus) type. The Rh type tells if a person does or does not have the Rh (also called D) antigen on their red cells. About 85% of people are Rh(D) positive, and 15% are Rh(D) negative.
Rh positive people, who have the D antigen on their cells, can receive Rh positive or Rh negative blood; it doesn’t matter to them. People who are Rh negative should receive only Rh negative blood. If Rh negative people have a transfusion of Rh positive blood, which carries the D antigen that they lack, their body will recognize that those blood cells do not belong to them. They may make anti-D antibodies to fight the foreign cells, as if they were dangerous bacteria or viruses.
There are actually more than three hundred blood group antigens* which may be present on human red cells; but the ABO and Rh(D) are the most important. People automatically make anti-A or anti-B if they lack those antigens, because of exposure to similar chemical groups in food or in the environment. The D antigen is next most likely to provoke antibodies in people who lack it, but only if they are exposed to D positive blood cells by transfusion or pregnancy. Once a person has these antibodies, they can destroy transfused red cells of the wrong type. Rh negative pregnant women receive a shot to prevent them from making anti-D, in case their baby is Rh positive; because if they made the antibody, it could also destroy their baby’s red cells before birth.
*Most blood group antigens are inherited differences in specific carbohydrate (sugar) or protein molecules on the surface of red cells.