Page 705 - Fundamentals of anatomy physiology

692 Unit 4 Fluids and Transport
In contrast, the plasma of an Rh-negative individual does
not contain anti-Rh antibodies. These antibodies are present
only if the individual has been sensitized by previous expo-
sure to Rh+ RBCs. Such exposure can occur accidentally during
a transfusion, but it can also accompany a seemingly normal
pregnancy involving an Rh– mother and an Rh+ fetus. (See
Spotlight Figure 19–8 on pp. 694–695.)
Cross-Reactions in Transfusions
When an antibody meets its specific surface antigen, the
RBCs agglutinate and may also hemolyze. This is called a
­cross-reaction (Figure 19–6b). For instance, an anti-A antibody
that encounters A surface antigens will cause the RBCs bearing
the A surface antigens to clump or even break up. Clumps and
fragments of RBCs under attack form drifting masses that can
plug small blood vessels in the kidneys, lungs, heart, or brain,
damaging or destroying affected tissues. Such cross-reactions,
19 or transfusion reactions, can be prevented by making sure that the
blood types of the donor and the recipient are compatible—
that is, that the donor’s blood cells and the recipient’s plasma
will not cross-react.
In practice, the surface antigens on the donor’s cells are more as hemolytic disease of the newborn (HDN). HDN involves Rh
important in determining compatibility than are the antibod-
ies in the donor’s plasma. Why? Unless large volumes of whole
blood or plasma are transferred, cross-reactions between the
donor’s plasma and the recipient’s blood cells will not produce
significant agglutination. This is because the donated plasma is
diluted quickly through mixing with the larger plasma v­ olume
of the recipient. (One unit of whole blood, 500 mL, contains
roughly 275 mL of plasma, only about 10 percent of normal
plasma volume.) When the goal is to increase the blood’s
oxygen-carrying capacity rather than its plasma volume, packed
RBCs, with a minimal amount of plasma, are often transfused.
This practice minimizes the risk of a cross-reaction.
Testing for Transfusion Compatibility
Extra care must be taken to avoid potentially life-threatening
cross-reactions between the donor’s cells and the recipient’s
plasma. For this reason, a compatibility test is usually per-
formed in advance. This process normally involves two steps:
(1) a determination of blood type and (2) a cross-match test.
The standard test for blood type considers only the three
surface antigens most likely to produce dangerous cross-
reactions: A, B, and Rh (Figure 19–7). The test involves taking
drops of blood and mixing them separately with solutions
containing anti-A, anti-B, and anti-Rh (anti-D) antibodies. Any
cross-reactions are then recorded. For example, if an individual’s
RBCs clump together when exposed to anti-A and to anti-B an-
tibodies, the individual has type AB blood. If no reactions occur
after exposure, that person must have type O blood.
The presence or absence of the Rh surface antigen is also
noted, and the individual is classified as Rh positive or Rh
Figure 19–7 Blood Type Testing. Test results for blood samples
from four individuals. Drops are mixed with solutions containing
antibodies to the surface antigens A, B, AB, and D (Rh). Clumping
occurs when the sample contains the corresponding surface
antigen(s). The individuals’ blood types are shown at right.
Anti-A Anti-B Anti-D Blood
type
A+
B+
AB+
O–
negative on that basis. Type O+ is the most common blood
type. The RBCs of type O+ individuals lack surface antigens A
and B but have the Rh antigen.
Spotlight Figure 19–8 describes a serious condition known
incompatibility between a pregnant mother and her develop-
ing fetus.
Standard blood-typing of both donor and recipient can
be completed in a matter of minutes. However, in an emer-
gency, there may not be time for preliminary testing. For
example, a person with a severe gunshot wound may require
5 liters or more of blood before the damage can be repaired.
Under these circumstances, type O blood (preferably O–)
will be administered. Because the donated RBCs lack both
A and B surface antigens, the recipient’s blood can have
anti-A antibodies, anti-B antibodies, or both and still not
cross-react with the donor’s blood. Because cross-reactions
with type O blood are very unlikely, type O individuals are
sometimes called universal donors. Type AB individuals were
once called universal recipients, because they lack anti-A and
anti-B antibodies that would attack donated RBCs, and so can
safely receive blood of any type. However, now that blood
supplies are usually adequate and compatibility testing is
regularly done, the term has largely been dropped. If the
recipient’s blood type is known to be AB, type AB blood will
be administered.
It is now possible to use enzymes to strip off the A or B sur-
face antigens from RBCs and create type O blood in the labora-
tory. The procedure is expensive and time-consuming and has