A2096 1..7

Incompatibility in the context of blood groups is due to the binding of plasma antibody with red cell antigen, thereby causing a reaction. In laboratory tests this reaction is most commonly visualized by agglutination of the red cells. In the body, an antigen–antibody reaction can occur as an adverse consequence of blood transfusion or pregnancy, resulting in accelerated red cell destruction. It is therefore important to detect incompatibility between the plasma of a patient and the red cells of a potential blood donor before transfusion, to avoid a transfusion reaction. Incompatibility occurs in pregnancy when the mother is immunized by cells of the fetus which cross the placenta. This immunization only occurs when the fetus has inherited a blood group antigen from the father which is ‘foreign’ to the mother. By far the most important blood groups in relation to blood transfusion are those within the ABO system and, in pregnancy, the D antigen of the Rh system. Antibodies may be naturally occurring or immune in type. The term ‘naturally occurring’ is used for blood group antibodies produced in individuals who have never been transfused with red cells carrying the relevant antigen or been pregnant with a fetus carrying the relevant antigen. Explanations for the existence of these antibodies include the possibility that some cells may be capable of making specific antibody in the absence of antigenic stimulus or that antibodies, such as anti-A and anti-B, may be produced as an immune response to substances in the environment which are antigenically similar to human blood group substances. An example might be glycoproteins on the surface of bacteria present in the gut, some of which are antigenically similar to the A and B antigens. Immune antibodies are those produced in response to antigenic stimulus by a foreign (nonself) antigen as a result of either blood transfusion or pregnancy.Any blood group can stimulate immune antibody production but those of the Rh system are the most common, notably anti-D. Antibodies that are capable of causing an adverse reaction to transfusion or are harmful to a fetus are said to be clinically significant. Blood group antigens are inherited characters which are detected by specific antibodies and may be protein or carbohydrate in nature. The blood groups that will be referred to in this text are those intrinsic to the surface of red cells, although some are present on other cells and tissues of the body. The genetic mechanism involved in the production of protein and carbohydrate antigens is different. Protein antigens (i.e. Rh) are direct products of the appropriate gene. However, the genes controlling the attachment of an immunodominant sugar to a cell membrane component encode transferase enzymes. Therefore, carbohydrate antigens (i.e. ABO) are indirect products of the defining gene. Some of the blood groupactive proteins in the red cell membrane are shown in Figure 1. There are over 250 blood group antigens which belong to one of the 25 genetically independent blood group systems but only those with the most clinical relevance, ABO and Rh, will be described in this article.