Defense Mechanisms of the Body
673
Type A individuals always have anti-B antibodies in
their plasma. Similarly, type B individuals have plasma anti-A
antibodies. Type AB individuals have neither anti-A nor anti-
B antibody, and type O individuals have both. These anti-
erythrocyte antibodies are called natural antibodies. How
they arise naturally—that is, without exposure to the appro-
priate antigen-bearing erythrocytes—is not clear.
With this information as background, we can predict
what happens if a type A person is given type B blood. There
are two incompatibilities: (1) the recipient’s anti-B antibodies
cause the transfused cells to be attacked, and (2) the anti-A
antibodies in the transfused plasma cause the recipient’s cells
to be attacked. The latter is generally of little consequence,
however, because the transfused antibodies become so diluted
in the recipient’s plasma that they are ineffective in inducing
a response. It is the destruction of the transfused cells by the
recipient’s antibodies that produces the problem.
Similar analyses show that the following situations would
result in an attack on the transfused erythrocytes: a type B per-
son given either A or AB blood; a type A person given either
type B or AB blood; a type O person given A, B, or AB blood.
Type O people are, therefore, sometimes called universal donors,
whereas type AB people are universal recipients. These terms are
misleading, however, because besides antigens of the ABO sys-
tem, there are a host of other erythrocyte antigens and plasma
antibodies against them. Therefore, except in a dire emergency,
the blood of donor and recipient must be tested for incompat-
ibilities directly by the procedure called
cross-matching
.
The
recipient’s serum is combined on a glass slide with the pro-
spective donor’s erythrocytes (a “major” cross-match), and
the mixture is observed for rupture (hemolysis) or clumping
(agglutination) of the erythrocytes; this indicates a mismatch.
In addition, the recipient’s erythrocytes can be combined with
the prospective donor’s serum (a “minor” cross-match), look-
ing again for mismatches.
Another group of erythrocyte membrane antigens of
medical importance is the Rh system of proteins. There are more
than 40 such antigens, but the one most likely to cause a prob-
lem is called Rh
o
, known commonly as the
Rh factor
because it
was fi rst studied in rhesus monkeys. Human erythrocytes either
have the antigen (Rh-positive) or lack it (Rh-negative). About
85 percent of the U.S. population is Rh-positive.
Antibodies in the Rh system, unlike the “natural anti-
bodies” of the ABO system, follow the classical immunity
pattern in that no one has anti-Rh antibodies unless exposed
to Rh-positive cells from another person. This can occur if
an Rh-negative person is subjected to multiple transfusions
with Rh-positive blood, but its major occurrence involves the
mother-fetus relationship. During pregnancy, some of the fetal
erythrocytes may cross the placental barriers into the mater-
nal circulation. If the mother is Rh-negative and the fetus is
Rh-positive, this can induce the mother to synthesize anti-Rh
antibodies. This occurs mainly during separation of the pla-
centa at delivery. Thus, a fi rst Rh-positive pregnancy rarely
offers any danger to the fetus because delivery occurs before
the mother makes the antibodies. In future pregnancies, how-
ever, these antibodies will already be present in the mother
and can cross the placenta to attack and hemolyze the eryth-
rocytes of an Rh-positive fetus. This condition, which can
cause an anemia severe enough to cause the death of the fetus
in utero or of the newborn, is called
hemolytic disease of the
newborn
.
The risk increases with each Rh-positive pregnancy
as the mother becomes more and more sensitized.
Fortunately, this disease can be prevented by giving an
Rh-negative mother human gamma globulin against Rh-
positive erythrocytes within 72 h after she has delivered an
Rh-positive infant. These antibodies bind to the antigenic
sites on any Rh-positive erythrocytes that might have entered
the mother’s blood during delivery and prevent them from
inducing antibody synthesis by the mother. The administered
antibodies are eventually catabolized.
You may be wondering whether ABO incompatibili-
ties are also a cause of hemolytic disease of the newborn. For
example, a woman with type O blood has antibodies to both
the A and B antigens. If her fetus is type A or B, this theoreti-
cally should cause a problem. Fortunately, it usually does not,
partly because the A and B antigens are not strongly expressed
in fetal erythrocytes and partly because the antibodies, unlike
the anti-Rh antibodies, are of the IgM type, which do not
readily cross the placenta.
Allergy (Hypersensitivity)
Allergy
,
or
hypersensitivity
,
refers to diseases in which immune
responses to environmental antigens cause infl ammation and
damage to the body itself. Antigens that cause allergy are
called
allergens;
common examples include those in ragweed
pollen and poison ivy. Most allergens themselves are relatively
or completely harmless—it is the immune responses to them
Table 18–9
Human ABO Blood Groups
Genetic Possibilities
Blood Group
Percent*
Antigen on RBC
Homozygous
Heterozygous
Antibody in Blood
A4
2
A
A
A
A
O
A
n
t
i
-
B
B1
0
B
B
B
B
O
A
n
t
i
-
A
AB
3
A and B
AB
Neither anti-A nor anti-B
O
45
Neither A nor B
OO
Both anti-A and anti-B
*In the United States
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