Chap10 - CHAPTER 10 BLOOD GROUPS: ABO AND Rh The success of...

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77 CHAPTER 10 BLOOD GROUPS: ABO AND Rh The success of human blood transfusions requires compatibility for the two major blood group antigen systems, namely ABO and Rh . The ABO system is defined by two red blood cell antigens, A and B, whose presence or absence is determined by three alleles (A, B, O) segregating at a single genetic locus. An unusual feature of this system is the presence of serum IgM antibodies in healthy adults to whichever antigen (A or B) is absent from that individual's cells. The presence or absence of Rh antigens on red blood cells is determined by two alleles at another locus, Rh. Rh INCOMPATIBILITY between mother and infant may result in ERYTHROBLASTOSIS FETALIS, which can be prevented by passive immunization of the mother with anti-Rh antibodies (RHOGAM). Many lives are saved throughout the world each year through the use of blood transfusions, by preventing death from loss of blood due to trauma, and by allowing performance of surgical procedures which would otherwise be impossible. However, it was not until the early nineteen hundreds that routine blood transfusion between humans became possible, following the discovery that genetically determined differences exist between the blood of different individuals. These differences must be identified and compatibility ensured before transfusions can be safely carried out. Many genetic systems controlling blood groups in humans are known, and they have been of considerable importance in our understanding of human genetics, as well as in both clinical and forensic medicine. We will discuss briefly the major features of the two most important of these systems, those controlling the ABO blood groups and the Rh factor . ABO BLOOD GROUPS A and B antigens The ABO blood groups are defined by the presence of two alternative antigens on red blood cells, determined by three alternative alleles at a single genetic locus. Two basic rules governing this system are as follows: 1) The blood "type" is defined by the presence of two red blood cell antigens, "A" and "B." RBCs of type A have the A antigen on their surface, those of type B have antigen B, type AB red cells bear both antigens, while type O cells bear neither antigen. 2) "Natural" antibodies called isoagglutinins exist in an individual's serum, directed against whichever of the A and B antigens is not present on that person's red cells (we will examine the source of these antibodies later).
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78 The presence of ABO antigens and antibodies ( isoagglutinins) in the four blood types is summarized below: BLOOD RBC SERUM TYPE ANTIGENS ANTIBODIES FREQUENCY A A anti-B 40% B B anti-A 10% AB A and B none 5% O none anti-A and anti-B 45% The success of blood transfusions depends on ensuring the compatibility of the blood types between donor and recipient. If the recipient has antibodies to the infused red cells, these red cells will be rapidly destroyed, resulting in a potentially lethal transfusion reaction . Type A blood given to a type B recipient, for instance, can result in such a reaction, since the
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This note was uploaded on 01/16/2012 for the course BI 200 taught by Professor Potter during the Fall '11 term at Montgomery College.

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Chap10 - CHAPTER 10 BLOOD GROUPS: ABO AND Rh The success of...

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