Uses for Recombinant Protein Drugs - Biotechnology and Your Health: Pharmaceutical Applications - page 68

share another quality—they don't stay around for a very long time.
Sometimes they die in the work they do, but they have a short life
expectancy in any case. This may be good from a biological point
of view, but it can also cause trouble. Red blood cells live for about
120 days, and white blood cells , the first responders to an infection,
only live for 1 to 1 1/2 days. This means that we must constantly be
able to replace blood cells, and we can. The bone marrow , the soft
tissue inside the larger bones in our body, manufactures these cells
and is normally very good at it. For example, every day, the bone
marrow produces about 200 billion red blood cells to replace those
that are dying. Having too few blood cells creates problems and
so does having too many, so our bodies have a system of growth
factor proteins that instruct the bone marrow to produce more of
a particular type of blood cell when and only when its numbers
drop. Many different bone marrow growth factors have been
found. Each works by docking at a specific receptor protein on an
immature bone marrow cell and triggering it to divide and develop
into the next stage, resulting in the production of the type of
cell the body needs. This system of matched signal proteins and
receptors is finely controlled, and the supply of cells normally
keeps up with the body's demand.
If something happens to cause a dramatic drop in the number
of blood cells—a drop too large to wait for normal production to
kick in—then a dose of the appropriate blood cell growth factor
might help. This is where biotechnology steps in. These proteins,
which trigger the cell division and the specialization of cells
(Figure 5.1), are present at very low levels in the blood. We have
seen in the discussion of hemophilia how trying to replace a
blood protein can lead to different kinds of problems. Blood cell
growth factors are present in very small amounts in the blood, so
purifying them from blood is impractical. Instead, scientists have
isolated the genes that direct the production of a number of these
blood-cell growth proteins, engineered them into an appropriate
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