Chapter 17
Blood
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17
surfaces, which identify each of us as unique from all others.
Tese glycoprotein markers are called
antigens.
An antigen is any-
thing the body perceives as foreign and that generates an immune
response. Examples are toxins and molecules on the surfaces of
bacteria, viruses, and cancer cells—and mismatched RBCs.
One person’s RBC proteins may be recognized as foreign if
transfused into someone with a different red blood cell type, and
the transfused cells may be agglutinated (clumped together) and
destroyed. Since these RBC antigens promote agglutination, they
are more specifically called
agglutinogens
(ag
0
loo-tin
9
o-jenz).
At least 30 groups of naturally occurring RBC antigens
(blood groups) are found in humans, and many variants oc-
cur in individual families (“private antigens”) rather than in the
general population. Te presence or absence of various antigens
allows a person’s blood cells to be classified into each of these
different blood groups. Antigens determining the ABO and Rh
blood groups cause vigorous transfusion reactions (in which the
foreign erythrocytes are destroyed) when they are improperly
transfused. For this reason, blood typing for these antigens is
always done before blood is transfused.
Other antigens (such as those in the MNS, Duffy, Kell, and
Lewis groups) are mainly of legal or academic importance. Be-
cause these factors rarely cause transfusion reactions, blood is
not specifically typed for them unless the person is expected to
need several transfusions, in which case reactions are more likely
to occur. Here we describe only the ABO and Rh blood groups.
ABO Blood Groups
Te
ABO blood groups
are based on the
presence or absence of two agglutinogens, type A and type B
(Table 17.4)
. Depending on which of these a person inherits,
his or her ABO blood group will be one of the following: A, B,
AB, or O. Te O blood group, which has neither agglutinogen,
is the most common ABO group in North America for whites,
blacks, Asians, and Native Americans. AB, with both antigens,
is least prevalent. Te presence of either the A or the B agglu-
tinogen results in group A or B, respectively.
Unique to the ABO blood groups is the presence in the plasma
of
preformed antibodies
called
agglutinins
. Te agglutinins act
against RBCs carrying ABO antigens that are
not
present on a
person’s own red blood cells. A newborn lacks these antibodies,
but they begin to appear in the plasma within two months and
reach adult levels between 8 and 10 years of age. As indicated in
±able 17.4, a person with neither the A nor the B antigen (group
O) possesses both anti-A and anti-B antibodies, also called
a
and
b agglutinins
respectively. Tose with group A blood have
anti-B antibodies, while those with group B have anti-A anti-
bodies. AB individuals have neither antibody.
Rh Blood Groups
Tere are 52 named Rh agglutinogens, each
of which is called an
Rh factor
. Only three of these, the C, D,
and E antigens, are fairly common. Te Rh blood typing system
is so named because one Rh antigen (agglutinogen D) was origi-
nally identified in
rhesus
monkeys. Later, the same antigen was
discovered in humans.
About 85% of Americans are Rh
1
(Rh positive), meaning
that their RBCs carry the D antigen. As a rule, a person’s ABO
and Rh blood groups are reported together, for example, O
1
,
A
2
, and so on.
infected by the hepatitis virus and, beginning in the early 1980s,
by HIV, a blood-transmitted virus that depresses the immune sys-
tem and causes AIDS. (See Chapter 21.) New infections are now
avoided as a result of new testing methods for HIV, availability of
genetically engineered clotting factors, and hepatitis vaccines.
Disseminated Intravascular Coagulation (DIC)
DIC is a situation in which widespread clotting occurs in intact
blood vessels and the residual blood becomes unable to clot.
Blockage of blood flow accompanied by severe bleeding follows.
DIC most commonly happens as a complication of pregnancy
or a result of septicemia or incompatible blood transfusions.
Check Your Understanding
9.
What are the three steps of hemostasis?
10.
What is the key difference between fibrinogen and fibrin?
Between prothrombin and thrombin? Between most factors
before and after they are activated?
11.
Which bleeding disorder results from not having enough
platelets? From absence of clotting factor VIII?
For answers, see Appendix H.
Transfusion and
Blood Replacement
Describe the ABO and Rh blood groups. Explain the basis of
transfusion reactions.
Describe fluids used to replace blood volume and the
circumstances for their use.
Te human cardiovascular system minimizes the effects of
blood loss by (1) reducing the volume of the affected blood
vessels, and (2) stepping up the production of red blood cells.
However, the body can compensate for only so much blood loss.
Losing 15–30% causes pallor and weakness. Losing more than
30% of blood volume results in severe shock, which can be fatal.
Transfusing Red Blood Cells
Whole blood transfusions
are routine when blood loss is rapid
and substantial. In all other cases, infusions of
packed red cells
(whole blood from which most of the plasma and leukocytes
have been removed) are preferred for restoring oxygen-carrying
capacity. Te usual blood bank procedure involves collecting
blood from a donor and mixing it with an anticoagulant, such as
certain citrate or oxalate salts, which prevents clotting by bind-
ing calcium ions. Te shelf life of the collected blood at 4°C is
about 35 days. Because blood is such a valuable commodity, it
is most o²en separated into its component parts so that each
component can be used when and where it is needed.
Human Blood Groups
People have different blood types, and transfusion of incompat-
ible blood can be fatal. RBC plasma membranes, like those of
all body cells, bear highly specific glycoproteins at their external
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