lymphatic tissue. But as I said previously,
they're evidence of an immune response.
When you look at the lymphoid nodules or the
lymphoid tissue you can see here, you can
see a germinal center, a pale staining area.
This pale staining area represents lymphocytes
undergoing division originating from a lymphoblast
in response to an antigen having been identified
by a lymphocyte that reverts into being a
lymphoblast and then producing an enormous
number of lymphocytes that have the capacity
to combat the invading pathogen. They differentiate
into B cells and plasma cells that secrete
antibodies. T cells are also present.
And I discussed the roles of these cells in a
lecture on the immune system or the lymphatic
tissues. Above these lymph nodules,
as we call them,
are M cells and other enterocytes. M cells
are just a very specialized enterocyte.
They actually endocytose or ingest pathogens or
antigens and then pass them out into the interstitial
space beneath them where they can be readily
identified by lymphocytes. And those lymphocytes
then go into the lymphoid nodules next to
them or in their vicinity, and they alert
the immune system to mounting an immune response,
and they'll return back to their location
and create these nodules shown here.
This slide has a diagram explaining what's
happening in these Peyer's patches or lymph
nodules. On the left-hand side, just make
sure you review the structure of the wall
of this small intestine shown here in this
histological image. And on the right-hand
side, there is an explanation of what's
going on, and I just briefly want to summarize
it. Lymphocytes travel through the blood and
they can leave the blood in the various peripheral
tissues by passing out of postcapillary high
endothelial venules. These are very specialized
epithelial cells that line these capillaries,
endothelium, that actually put little flags
up and attract lymphocytes into the underlying
tissue when there's an inflammatory situation
going on, or when there's an invading pathogen.
They sense what's going on and they alert
lymphocytes to attach to their endothelial
wall and then move into the system,
into the tissues.
And then those lymphocytes can hunt around
and detect antigens to which they're trying
to detect, and then mount the response that
you see here that I explained earlier.
And then they can secrete antibodies, having differentiated
into plasma cells, or they can become memory
cells and then spread to the rest of the body
where they can then strategically locate themselves
to identify future invasions of these pathogens
and then react a lot quicker, or there could
be T cells moving through to do their role.
And they leave this area through efferent
lymphatic vessels that are labelled here in
yellow. There is no afferent supply, lymph
supply to this area. This is where lymph vessels
originate. They originate carrying lipids
from lacteals in the lamina propria I'll
mention in a moment.
You have seen this slide before earlier in
the lecture. But the reason why I'm showing
it to you now is because I want you to focus
on the brush border that's in bold labelled
here on the left-hand histological section.
You can see it with the light microscope.
And you can also see details in the electron
microscope. So I'd like to just briefly
explain the importance of this brush border.
On the right-hand side of this slide is an