547
CHAPTER FOURTEEN
Blood
About 15% of Caucasians and 5% of African Americans in
the U.S. population are Rh-negative. The remaining ethnic
groups are all Rh-positive.
As in the case of antigens A and B, the presence (or
absence) of Rh antigens is an inherited trait. Antibodies for
Rh (
anti-Rh
) form only in Rh-negative persons in response to
the presence of red blood cells with Rh antigens.
If an Rh-negative person receives a transfusion of
Rh-positive blood, the recipient’s antibody-producing cells
are stimulated by the presence of the Rh antigens and will
begin producing
anti-Rh antibodies.
Generally, no seri-
ous consequences result from this initial transfusion, but
if the Rh-negative person—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. If this is her
F
rst pregnancy with an Rh-positive fetus, it may be unevent-
ful; however, at the time of this infant’s birth (or if a mis-
carriage 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
(f
g. 14.23)
.
If a woman who has already developed anti-Rh anti-
bodies 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
(e
˘rith
ro-blas-to
sis fe
tal-iz), or hemolytic disease of the
preferred blood types for normal transfusions and permis-
sible blood types for emergency transfusions.
PRACTICE
49
Distinguish between antigens and antibodies.
50
What is the main concern when blood is transfused from one
individual to another?
51
Why is a type AB person called a universal recipient?
52
Why is a type O person called a universal donor?
When is type O blood not really type O blood? This occurs when a cer-
tain mutation is present. A person with a rare genetic condition called
the Bombay phenotype lacks an enzyme that inserts a particular mol-
ecule onto red blood cell surfaces. Without that molecule, 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 phenotype does not aF
ect health,
it can sometimes explain a child’s ABO type that cannot be derived
from those of the parents.
Rh Blood Group
The
Rh blood group
was named after the rhesus monkey in
which it was F
rst studied. In humans, this group includes
several Rh antigens (factors). The most prevalent of these
is
antigen D:
however, if any of the antigen D and other Rh
antigens are present on the red blood cell membranes, the
blood is said to be
Rh-positive.
Conversely, if the red blood
cells lack the Rh antigens, the blood is called
Rh-negative.
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Rh-negative
woman with
Rh-positive
fetus
Cells from
Rh-positive
fetus enter
woman’s
bloodstream
In the next
Rh-positive
pregnancy,
maternal
antibodies
attack fetal red
blood cells
Woman
becomes
sensitized—
antibodies (
+
)
form to fight
Rh-positive
blood cells
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FIGURE 14.23
Rh incompatibility. If a man who is Rh-positive and a woman who is Rh-negative conceive a child who is Rh-positive, the woman’s
body may manufacture antibodies that attack future Rh-positive oF
spring.
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