O

What is the main concern when blood is transfused from one individual to another?

Why is a type AB person called a universal recipient? Why is a type O person called a universal donor?

When is type O blood not really type O blood? Blood typing involves adding known antibodies to a blood sample to see if the blood will clump, demonstrating the presence of corresponding antigens on the red blood cell membranes. A person with a rare genetic condition called the Bombay phenotype lacks an enzyme that inserts a particular sugar onto red blood cell surfaces. Without that sugar, the A and B antigens cannot bind. The result is blood that tests as O (because it lacks A and B antigens) but can genetically be of any ABO type—A, B, AB, or O. Although the Bombay phe-notype does not affect health, it can sometimes explain a child's ABO type that cannot be derived from those of the parents.

and will begin producing anti-Rh antibodies. Generally, no serious consequences result from this initial transfusion, but if the Rh-negative person—who is now sensitized to Rh-positive blood—receives another transfusion of Rh-positive blood some months later, the donated red blood cells are likely to agglutinate.

A related condition may occur when an Rh-negative woman is pregnant with an Rh-positive fetus for the first time. Such a pregnancy may be uneventful; however, at the time of this infant's birth (or if a miscarriage occurs), the placental membranes that separated the maternal blood from the fetal blood during the pregnancy tear, and some of the infant's Rh-positive blood cells may enter the maternal circulation. These Rh-positive cells may then stimulate the maternal tissues to begin producing anti-Rh antibodies (fig. 14.23).

If a woman who has already developed anti-Rh antibodies becomes pregnant with a second Rh-positive fetus, these anti-Rh antibodies, called hemolysins, cross the pla-cental membrane and destroy the fetal red cells. The fetus then develops a condition called erythroblastosis fetalis (hemolytic disease of the newborn). Clinical Application 14.5 discusses human blood transfusions and substitutes.

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