S e le cte d B lood Te s ts
OBJECTIVES
1. Define and understand the relationships between the key terms listed in this exercise.
2. Use the appropriate antisera to determine an unknown blood type.
3. Interpret the results of a blood typing experiment.
4. Determine who can donate blood to and receive blood from an individual of a particular blood type.
5. Explain the experimental method of preparing a hematocrit sample.
6. Determine the hematocrit of a blood sample via two different methods.
Blood tests provide a wealth of information on not only the diseased state, but the healthy state as well.
Today, you’ll be introduced to two common and important blood tests: blood typing and the hematocrit.
Blood Typing
The human body contains approximately 100 trillion cells. Each cell is marked by unique combinations of
carbohydrates, proteins, and other chemicals that inform the immune system that the cell belongs. Such
cellular markers are referred to as antigens. The body contains about 30 trillion red blood cells and, not
surprisingly, these erythrocytes contain antigens on their surface. Each RBC has about 50 antigens that
stud its cell membrane. We’ll only be concerned with three of these antigens: the A antigen, the B
antigen, and the D antigen. The presence or absence of these three antigens on an individual’s RBCs
determines his or her blood type.
If an individual’s RBCs contain the A antigen, the individual will be blood type A. If the RBCs contain
the B antigen, the individual will be blood type B. If each RBC contains both the A and the B antigens,
the individual will be blood type AB. If neither the A antigen nor the B antigen is present on the RBCs,
the individual is blood type O.
Figure 2-1. ABO blood types.
The D antigen is also referred to as the Rh antigen or the Rh factor. If it is present on the RBCs, the
individual is said to be Rh positive. If the RBCs lack the D antigen, the individual is said to be Rh
negative. For example, if an individual’s RBCs contain both the A and the D antigen, his blood type is A
positive (A+). An individual whose RBCs contain the B antigen but lack the D antigen is type B negative
(B–).
The presence of such self-antigens allows the immune system to recognize the body’s normal cells. If the
immune system detects foreign antigens, it will typically attempt to destroy them. One method of
destruction involves specialized immune proteins known as antibodies. Normally, the body makes
antibodies when exposed to a particular antigen. For example, one will not have antibodies to the chicken
pox virus unless one has been exposed to it. However, there is an important exception to this rule. About
six months or so after birth, one begins to produce antibodies that target the RBC antigens that one lacks.
For example, because someone with blood type A would lack the B antigen, their plasma would contain
anti-B antibodies. The plasma of someone with blood type B would contain anti-A antibodies.
Figure 2-2. Antigens and antibodies.
The plasma of someone with blood type AB would contain neither anti-A nor anti-B antibodies. Finally,
the plasma of someone with blood type O would contain both anti-A and anti-B antibodies. Because the
anti-A and anti-B antibodies are made without any exposure to the A and B antigens, they are referred to
as preformed antibodies.
ABO
Blood
Type
RBCs Contain
Plasma Contains
A
A antigens
Anti-B antibodies
B
B antigens
Anti-A antibodies
A and B antigens
Neither anti-A nor anti-B
antibodies
Neither A nor B antigens
Both anti-A and anti-B antibodies
AB
O
Figure 2-3. ABO antigen-antibody complex.
It’s important to note that the anti-D antibody is not preformed. It is made by Rh negative individuals
only if they are exposed to the Rh factor.
The basic antibody looks somewhat like the letter Y and can grasp two antigens at one time. This allows
antibodies to cross-link antigens with one another and create a clump of antigens. The clumping process
is referred to as agglutination. Unwanted agglutination is not a good thing. Clumps can block blood
vessels and even cause death! (However, the agglutination of foreign antigens increases their likelihood
of phagocytosis.)
Figure 2-4. Agglutination.
It is imperative that agglutination is avoided when blood transfusions take place. In order to prevent
transfusion reactions, the recipient’s antibodies must not be able to attack the donor’s antigens. Let’s first
consider the ABO blood types.
Blood Type
Can Donate To
Can Receive From
A
A and AB
A and O
B
B and AB
B and O
AB
AB, A, B, and O
O, A, B, and AB
O
AB
O
Notice that you can always give to and receive from someone with the same blood type. Also, note that
individuals with blood type AB can receive blood from all four types, while individuals with type O can
give to all four types. Individuals with type AB are the universal recipients because they lack both the
anti-A and the anti-B antibodies. Individuals with type O are the universal donors because they lack A
and B antigens.
Examine this diagram representing safe transfusions between the ABO blood groups.
Now let’s consider the D antigen.
Blood Type
Can Donate To
Can Receive From
Rh Positive
Rh+
Rh+ and Rh–
Rh Negative
Rh– and Rh+
Rh–
Note that the only prohibited transfusion is Rh+ individuals donating to Rh– individuals. The immune
system of the Rh– individual would see the Rh factor as being foreign and produce anti-Rh antibodies to
destroy it.
Examine this diagram depicting safe transfusions when considering the D antigen.
Also note that we should modify our designation of the universal donor and recipient. The universal
donor is more accurately type O– and the universal recipient is type AB+.
ACTIVITY 1
You will be given four samples of imitation blood. Your task will be to determine the blood type of each
sample. You will also have at your disposal three colored anti-serasolutions. One solution contains anti-A
antibodies. One contains anti-B antibodies. And finally, one contains anti-Rh antibodies. You will also
have four plastic trays (one for each blood sample). Each tray contains three wells, labeled A, B, and Rh.
Set a tray on a white piece of paper and add two drops of antibody-A solution in the A well, two drops of
antibody-B solution in the B well, and two drops of the antibody-Rh in the Rh well. Next add two drops
of one of your blood samples to each well. Use a toothpick to gently mix the blood and the antibody
solution. (NB: Make sure you use each toothpick only once so as to avoid any contamination.) Repeat the
process using a new tray for each blood sample. Make sure you are using a separate tray for each blood
sample.
You will now examine each well for signs of agglutination, i.e., the appearance of reddish granules. The
occurrence of agglutination in a well tells us that an antigen is present. Determining the presence of
antigens lets us discern the blood type. Let’s consider an example.
Recall that you initially placed anti-A antibodies into the A well. Suppose agglutination is visible in the A
well after the addition of the blood. That indicates that the anti-A antibodies must have cross-linked A
antigens. Thus, the blood sample must contain A antigens. We can now assume that the particular blood
sample is type A or type AB, since both contain A antigens. The next step is to examine the B well.
Suppose agglutination is visible in it as well. This means that the blood sample must also contain the B
antigen. Thus, we know the blood type is AB. To determine whether it is AB+ or AB–, we have to
examine the Rh well. Agglutination in the Rh well indicates that the blood sample contains Rh antigens,
and is type AB+. A lack of agglutination indicates an absence of the Rh factor and the blood type is AB–.
Once you’ve ascertained the blood type of each sample, fill out the chart below.
Sample
Blood
Type
Antigens
Antibodies
Donates
To
Receives
From
HEMATOCRIT
Blood is made of myriad components, including water, cells, proteins, electrolytes, and many others. One
way of examining any heterogeneous solution is to separate it into its different components. We’ll
examine one important method of fractionating blood.
A centrifuge is a machine that attains a very high speed of revolution. When a tube of blood is
centrifuged, its components separate by density. The heavier components will end up at the bottom of the
tube while the lighter components will be found at the top of the tube. Three layers appear in a sample of
centrifuged blood. However, usually only two of the three layers will be visible to the naked eye.
Normally, the material in the lower half of the tube will appear red, while the material in the upper
portion will have a yellowish hue. The red portion consists of packed RBCs (hence, the red color). The
yellow portion is the plasma. In between the RBCs and the plasma is a section of intermediate density that
will not be visible to the naked eye. This section is referred to as the buffy coat. The buffy coat consists of
white blood cells and platelets.
Figure 2-5. Centrifuged blood.
The percentage of the total blood volume that is occupied by the RBCs is referred to as the hematocrit.
The hematocrit provides an indication of the body’s oxygen-carrying ability. The average hematocrit for
males is 49 ±7%. The average hematocrit for females is 42 ±4%.
A decrease in the body’s oxygen-carrying ability is known as anemia. A low hematocrit is indicative of
anemia. An abnormally high RBC count causes an abnormally high hematocrit. Such a situation is
referred to as polycythemia. Somewhat paradoxically, the body’s ability to transport oxygen can decline
during polycythemia despite the relative abundance of RBCs. This is because the high RBC count
increases the blood’s viscosity, making blood flow sluggish.
ACTIVITY 2
You will be examining a “simulated” capillary tube containing blood. Your task will be to determine its
hematocrit, decide whether it is normal, and, if it is abnormal, suggest a possible cause.
There are two methods of determining hematocrit with which you should be familiar. First, you can
simply use a ruler. Measure the height of the total blood sample in the tube. Then measure the height of
the packed RBCs in the tube. Divide the packed RBC height by the total sample height. Multiply the
result by 100 and you have determined the hematocrit.
[EQUATION]
The second method requires the use of a CRITOCAPS card. Your instructor will demonstrate the
procedure.
Hematocrit
Normal/Abnormal
Explanation
Chapter 1 Review
Name
ID No.
Instructor
Course/Section
Partner’s Name (if applicable)
Date (of lab meeting)
1. Tom has anti-A and anti-D antibodies in his plasma. What is his blood type? What antigens are
present on his RBCs?
2. Lucy, who is blood type B–, just received a blood transfusion without complications. Suggest a
plausible blood type of her donor.
3. Manuel has blood type O+. Name two blood types from which he could receive blood.
4. Terri’s RBCs contain one type of antigen and her plasma contains two varieties of preformed
antibodies. What blood type does Terri have?
5. Briefly explain the difference between agglutination and coagulation.
6. Ed has a hematocrit of 34%. Is this normal? What term would be used to describe his condition? How
would this affect his ability to exercise?
7. Zoë has a hematocrit of 40%. Approximately what percentage of her blood volume is occupied by
plasma?
8. In which region of centrifuged blood would the most neutrophils be found?
9. In which region of centrifuged blood would the most albumin be found?
10. Consider the following results of the blood typing experiment described earlier. Note that shading
indicates agglutination. For each sample, fill out the accompanying chart.
Blood Type
Antigens
Present
Antibodies
Present
Donates To
Receives From
Blood Type
Antigens
Present
Antibodies
Present
Donates To
Receives From
Blood Type
Antigens Present
Antibodies
Present
Donates To
Receives From