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Narrator: Listen to part of a lecture in a biology class. The professor is discussing blood types.
Professor:
Before we begin our discussion of blood types, let’s review what we know about blood. According to the  Q23
textbooks, about half of the volume of blood is made up of blood cells that begin as stem cells in bone
marrow. And these stem cells can develop into any of the other kinds of cells found in blood, including
red cells, white cells, and platelets. So... some stem cells become white cells or leukocytes and these
are essential to the immune system. And when bacteria or germs invade the body, some of the white
cells form antibodies to resist the infection directly while other white cells begin to work on the chemistry
of the foreign substance itself... to fight the infection. Now, compared with red blood cells, there are
relatively few white blood cells... only about one for every seven hundred red cells. And the smallest of
the blood cells are called platelets, but what they don’t have in size they make up for in numbers. Well,
most of us have about two trillion of them and they work to help the blood to clot and  uh, repair
holes in the walls of blood vessels. But we need a way to transport the blood, right? Plasma is the liquid
substance in blood that transports most of the chemicals... vitamins and minerals... hormones and
enzymes.
    But most stem cells become red blood cells, or erythocytes. They’re the most numerous. As I
mentioned before, there are about seven hundred to every one white blood cell. So the red cells give blood
the color red, and they’re important for what we call blood typing. And blood typing is what I want to get  Q23
going on today.
    Now blood types are a classification of red blood cells according to the presence of specific
substances... antigenic proteins and carbohydrates... and you can see them under the microscope on
the surface of the cells. The four blood types are identified by letters... A, B, AB, and O. Blood type A
contains red blood cells with the antigen A. Blood type B contains red blood cells with the antigen B. The
AB blood type contains both antigens, and the O blood type contains no antigens, but the individual with
this type can form antibodies containing either A or B antigens.
Student 1:
Excuse me. Is that why the O blood type is considered a universal blood type? Because it can form  Q24
antibodies with either A or B antigens? Professor:
Bight you are. But, in typing the blood, the antigens are really much more complex than this explanation
might suggest. There are at least 300 different antigens, In fact, there are so many potential  Q25
combinations that an individual’s blood type is almost as unique as a fingerprint. But anyway, these basic types
are used for determining compatibilities for blood transfusions. Before a transfusion is approved,  
hospitals always perform a procedure called a cross match which involves taking a sample of the donor’s red
blood cells and mixing them with a sample of the patient’s plasma. You see, in almost every individual,
the plasma contains antibodies that will react to antigens that are not found on their own red blood ceils.
So during a transfusion, antibodies in the patient’s plasma can bind to antigens on the donor’s red blood
cells when the donor’s blood is not similar to that of the patient. Well, many minor reactions can occur
like fever or chills, but some reactions are so severe that they lead to a... a spontaneous destruction
of the red blood cells from the donor and that can result in shock or even death. So you can understand
why blood typing is so important. Cross matching lowers the risk of a serious reaction.
    Okay. In cross matching, we take red cells from one person and plasma from the other person, and
we watch to see whether there’s a negative response. Take a look at this diagram. It’s on page 112 in
your textbook. Here’s what you would be looking at with a reaction caused by incompatible blood. See
how the cells clump together?
The reason that this is happening is because there’s a chemical reaction between the protein molecules
in the red cells of one person and the plasma of the other. Now look at this slide. This diagram is on the  Q26
next page in your text and this shows a compatible match with no clumping
See how the cells are evenly spaced?
    Well, of course, doctors prefer to use the same type as that of the patient, but compromises have to
be made in emergencies. Type A patients can’t receive type B blood, and type B patients can’t receive
type A blood, but back to your question; since an O donor has blood that’s compatible with both A and
B antigens, it’s the ideal, or as you said, the universal donor. In an emergency, type O blood can be
used for patients of all blood types. And fortunately, worldwide, type O is the most common, followed by
type A. Relatively few people have type B blood, and the fewest have type AB.
Student 2: Professor Stephens, can you tell us anything about artificial blood?  Q27
Professor:
Well actually, scientists developed artificial blood that’s been used successfully in blood transfusions
with human patients. It’s a white fluid, chemically similar to Teflon, the material that coats cookware and
prevents material from adhering to it. The fluid can be used as a match with all blood types, and so, the
cross matching step in transfusions... that can be eliminated. So far, artificial blood has done a good
job of replacing the red cells by carrying oxygen through the body and eliminating carbon dioxide, but
there are no white cells present, no antibodies, no platelets. So, it doesn’t clot, and it doesn’t remain in the
body very long. Still, continuing research along these lines should probably be encouraged. Um, even  Q28
with cross matching and other precautions, transfusions with human blood involve risks.