BIOLOGY Volume 2 - A Guide to General Biology - 2004

14. TRANSPORT IN ANIMALS

14.9. The Immune System

14.9.8. The Rhesus Factor

Approximately 85% of the world's population carry an antigen known as the Rhesus factor (Rh) on their red Blood Cells; such individuals are referred to as Rhesus-positive (Rh⁺). The remainder of the population lacks this antigen and are therefore Rhesus-negative (Rh^-). The plasma of Rh-negative blood does not necessarily contain Rhesus agglutinins; however, if Rh⁺ blood enters the body of an Rh^- individual, Rhesus agglutinins will be produced.

The Rhesus status (like the ABO blood group system) is genetically determined, with the presence of Rh (Rh⁺) being a dominant trait and its absence (Rh^-) being recessive. The Practical significance of this factor becomes apparent when a Rhesus-negative mother is pregnant with a Rhesus-positive fetus. In the later stages of Pregnancy, fragments of the fetus's Rhesus-positive red blood cells can cross into the maternal bloodstream and stimulate The production of Rhesus Antibodies. These antibodies can, in turn, cross the Placenta and destroy the fetal red blood cells. During a first Rhesus-positive pregnancy, the mother's body generally does not have time to produce a sufficient quantity of antibodies, and the fetus does not suffer serious complications. However, in subsequent Rhesus-positive pregnancies, chronic destruction of red blood cells may occur. Unless Miscarriage happens, such infants are typically born premature and suffer from anemia and jaundice. In these cases, a complete replacement of the infant's blood with normal donor blood is necessary. This condition is known as hemolytic disease of the newborn (erythroblastosis fetalis). It can be fatal to the child, especially if born prematurely. Modern techniques allow for complete intrauterine blood transfusions, though timely screening of the mother and fetus, as discussed below, makes it possible to avoid this Procedure altogether.

Prevention of Rhesus Incompatibility

1. During a first Rhesus-positive pregnancy, Antibody production in a Rhesus-negative woman is particularly active during and immediately after childbirth. If she is given an injection of Rhesus antibodies (known as anti-D) within the first 72 hours postpartum, it will prevent her from becoming sensitized by the fetal Rhesus-positive red blood cells that entered her Circulation. The administered antibodies bind to the Rhesus Antigens and mask them, preventing the mother's own lymphocytes from recognizing the target and reacting to it by forming corresponding memory cells (Fig. 14.44). Clearly, such prophylaxis requires screening all high-risk groups—in this case, all pregnant women—for Rhesus compatibility. In the UK, for example, this is a mandatory obstetric procedure.

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Fig. 14.44. Rhesus incompatibility (hemolytic disease of the newborn) and its prevention.

2. If a Rhesus-negative woman with blood group O(I) carries a Rhesus-positive fetus with any blood group other than group O, no problems arise. This occurs because any fetal red blood cells entering the maternal circulation are destroyed by the circulating anti-a and anti-b antibodies in her blood before an immunological response to the Rhesus antigen can develop.