00:01
There are two major regions that
the immune cells are first of all derived, and
also where they’re located in most concentrations.
00:16
As I've mentioned earlier, the cells originally
derive from the bone-marrow. That’s where
they’re produced, or they can be produced
in the gut-associated lymphoid tissues, or
as I mentioned earlier, from the thymus. That’s
where the T cells, remember, are educated.
00:34
These cells can then go and locate in the
lymphoid organs that I’ll describe in this
lecture. The nodules, the nodes, tonsil, and
also the spleen. They’re all antigen-dependent,
and they are the effectors of T and B cells.
Once they’re switched on by an antigen,
they become effector lymphocytes. And as we’ll
see also, they remember so that they can mount
a response a lot quicker to a secondary invasion
of a particular antigen. So let’s now have
a look at some of these lymphatic tissues,
and also the lymphoid organs. Here are two
pictures, two images taken from a part of
the gut. One happens to be the stomach on
the left-hand side, the stomach mucosa. The one
on the right is taken from the small intestine,
the duodenum. And on the left-hand side, you
see what we call diffuse lymphatic tissue.
01:38
Lymphatic tissue is easily identified in mucosal
surfaces because you see lots and lots of
little tiny black dots. These represent lymphocytes.
They don’t have much cytoplasm around them
because they haven’t yet been activated.
They’re waiting there on the surveillance
duty. They are trying to recognize a particular
antigen. And when they find that antigen that
may have found its way across the surface
of the epithelium, they recognize it, they
bind to it, and then they can mount an immune
response against it. On the right-hand side,
you see evidence of that. When you see a nodule
that you see on the right-hand side, a nodule
is a pale staining germinal centre, and a darker
staining corona, that represents a lymphatic nodule.
02:32
That is the morphological or the histological
evidence that there is an immune response
going on, because what’s happening there is
that an antigen is passed across the surface.
02:46
It might have been ingested by an M cell or
an enterocyte, a specialized enterocyte which
is the M cell, and that might have ingested
the antigen and then released it into the
interstitial compartment underneath the epithelial
surface to be recognized by a particular lymphocyte.
03:07
And when that lymphocyte recognizes that antigen,
it binds to it, and it could immediately go
into a situation where it can differentiate into
a lymphoblast and then massively proliferate.
03:22
And that proliferation is evidence of that
lighter staining germinal centre. And these cells
can form memory cells or plasma cells. The
plasma cells will secrete antibody. The memory
cells are there to leave that area through
part of the lymphatic system, a very small
lymphatic channel, and then pass to a lymph node
and beyond and get into the general circulation.
03:54
When they go to the lymph node, they can be
detected and they can spark up another immune
response and produce more memory cells and
more plasma cells, and they in turn can then
move and populate other mucosal surfaces just
in case these invading antigens is coming
across another surface. They can go out and
be located for a long time sitting in these
mucosal surfaces waiting for that antigen
to come through. The corona you see there
is really just where the peripheral cells
are being pushed aside. They actually are
lymphocytes that have detected the specific
antigen that is being wrecked against in this
situation. So they’re just like lymphocytes,
as I’ve mentioned, that have been in the
area. But as all this activity, this
germinal centre gets bigger, they just get
pushed to this side. Here is a tonsil. On
the left-hand side, the tonsil sitting in
the oral cavity is a perfect spot to detect
any antigens that come in with the food.
05:02
They have tonsillar crypts. Tonsillar crypts are
invaginations of the epithelial surface into
the underlying connective tissue. And food
debris gets caught in those crypts and it
remains there for some time. And what happens
is the lymphocytes, an antigen presenting
cells, move across the epithelial surface into
that debris, and then undergo surveillance
to detect any antigens which they’re trying
to detect. And if they do, again, they mount
an immune response and you see a lymphatic nodule,
as you see in this slide on the right-hand
side. And again, these lymphocytes can always
move away into lymph nodes nearby, create
another lymphatic nodule, and again, proliferate
more and more B cells, and more and more plasma
cells, and again, more memory cells that can
go and populate other surfaces as I described
earlier. There is so much traffic across the
tonsil surface, the epithelium of the tonsil
that often, the epithelium is very, very difficult
to recognize. It’s a stratified squamous
epithelium, but as I said, in most areas where
there’s so much traffic, it’s difficult
to recognize that epithelial surface. You’ve
seen on the left-hand side lymphatic nodules.
06:28
This is gut-associated lymphoid tissue. It’s
very common in the ileum, part of the small
intestine, combating against any antigens
that found their way, passed all the usual
defenses we often have. There are defenses
we have where our first line of defense can
be mucous, can be lysozymes. It can be the secretions
on our skin surface, that's antimicrobial.
06:58
There's other ways in which we first try
to prevent any antigen from getting into the
body, the barriers. But if they do, then on
the left-hand side, you can see a response
against it. And on the right-hand side is
the appendix, which is totally populated by
lymphoid tissue. All those little dark dots
are lymphocytes. And if you look very carefully
in that slide, you can see nodules, again, reacting
to antigens in the lumen of the appendix.