or that antigen. Let us move on to the
agranulocytes, the lymphocytes.
I am going to talk about these cells later
on when we talk about the immune system.
They are very common in blood. They occupy about
28 to 30 percent. So when you look at blood
smears, you can see a number of this different
sorts of lymphocytes. When they are traveling
in blood, they are immunocompetent. But they
are waiting to be attracted and identify
antigens in tissues. So they actually continually
leave the blood through lymph nodes that I
will talk about in a later lecture. They circulate
through the lymph node, being immunocompetent
that recognized the antigens. They have
been designed to recognize and take care of
an antigen or at least initiate that immune
response against an antigen. If they don't
come across the antigens, they can leave the
lymph node and then they can get into the
blood system and recirculate through the body
again looking for these foreign antigens.
They are on surveillance duty, but they do
not do their job in blood. They only use blood
to transport themselves to different locations
in the body. There are very small lymphocytes.
There are very large lymphocytes. They range
from roughly 6 to 8 microns to about 18 microns
in size. A granulocyte that is very important
to understand is the monocyte. It is a huge
cell. It is about 18 to 20 microns in diameter
when you see it in a blood smear. Now again, they
are traveling through blood and they are not
doing anything within the blood itself.
It is only when they moved into connective
tissue that they do their job. They transform
into being a macrophage and we will learn
about these macrophages when we look at all
the tissues of the body and all the organs
of the body in more detail. They have a very
characteristic indented nucleus, a bean-shaped
nucleus and that enables them to be identified,
apart from their massive sizes well relative
to the other blood cells. Now again reflect
on what I've said earlier about the orientation
of cells in the blood smear. You are looking
at a whole cell here. If that monocyte
that was rotated in another direction,
you may not see that in indented nucleus, so it may
make it difficult to say that some monocyte
could, in fact, be a mast cells travel through
the blood and they look like monocytes.
It is only when they move into the connective
tissue, those mass cells then accumulate the
granules that they use in their role as being
vasoactive agents and mediators.