The secondary lymphoid tissues have structures that
support the activation of the adaptive immune response.
There are a whole variety of cells
within the secondary lymphoid tissues.
Different types of T-cells, B-cells,
the antibody secreting plasma
cells, dendritic cells which show antigen to T-cells, follicular
dendritic cells which show antigen to B-cells, macrophages and
natural killer cells; all of these cell types are present.
So really it’s the secondary lymphoid tissues
where the adaptive response becomes activated.
So let’s look at these different
types of secondary lymphoid tissue.
Lymph nodes are scattered
throughout the body.
Lymphocytes can circulate around the body in the
blood circulation and in the lymphatic circulation.
And these two different circulations are not isolated
from each other, there are connections between them.
Particularly important being the thoracic
duct which permits lymphocytes that
are circulating around the lymphatic
system to re-enter the blood circulation.
Lymph nodes throughout the body
are connected with one another via
the lymphatic circulation, and
they also all have a blood supply.
So antigen can enter the lymph node either through the
lymphatic vessels or potentially through the blood vessels.
And likewise, lymphocytes and other
cells of the immune response can enter
the lymph node either via the lymphatics
or via the blood circulation.
The outside of the lymph node is
surrounded by a collagenous capsule.
Trabeculae penetrate into the inside
of the lymph node from this capsule.
Underlying the capsule
is the subcapsular sinus.
The inner part of the lymph node
is referred to as the medulla.
And there is a sinus where cells
from the medulla and fluid from
the medulla can drain into, and this is the medullary sinus.
Antigen, particularly antigen being carried by dendritic cells,
can arrive in the lymph node via the afferent lymphatic vessel.
Lymphocytes can enter via the blood circulation, although
they can also enter through the afferent lymphatics.
In order to enter the cellular part of the lymph
node, the lymphocytes arriving via the arteries
will enter through specialized structures
referred to as high endothelial venules or HEV.
Once lymphocytes become activated within the lymph
node, they organize themselves into follicles.
And particularly the B- lymphocytes produce follicles
within the outer part of the lymph node, the cortex.
So the cortex constitutes the
B-cell zone of the lymph node.
Within the cortex, the B-cells will
eventually form into structures
called germinal centers that develop
in response to antigenic stimulation.
Cells can then exit from the lymph node following their
activation, by using the efferent lymphatic vessels.
Turning now to the spleen; the spleen
is comprised of red pulp, which contains
erythrocytes and white pulp which contains
the white blood cells of the immune system.
Trabecular arteries will
feed into the white pulp.
There’s a central arteriole and a follicular arteriole by which
lymphocytes can enter into the white
pulp from the blood circulation.
The outer area of the white pulp is
referred to as the marginal zone.
And there are marginal sinuses from
which cells and molecules can drain.
The T-cell zone is referred to as the
periarteriolar lymphoid sheath or PALS, P-A-L-S.
And the B-cells, again organize themselves into
follicles, just like we saw in the lymph node.
Here we can see a tissue section of
spleen and we can clearly identify the
periarteriolar lymphoid sheath, a sheath
of lymphocytes surrounding the artery.
Following antigenic stimulation just like in the
lymph node, germinal centers can be produced.
Here in this immunofluorescent stained
section, we can see the T-cell
zone; here an antibody against the
T-cell surface molecule CD3, an
antibody coupled with a red fluorescent
dye is identifying where the T-cells
predominantly are located within the
periarteriolar lymphoid sheath.
Whereas here, using an antibody against the cell
surface molecule on the surface of B-lymphocytes,
the CD20 molecule, you can see that there is a
clear B-cell zone with a lymphoid follicle.
And finally, the Mucosa-Associated
These as their names suggests,
are found at mucosal surfaces.
Interspersed amongst the mucosal epithelial cells
are specialized M cells, or Microfold cells.
These contain pockets in which immune system cells, T-cells,
B-cells, macrophages, dendritic cells are located.
Underlying the M cells, will be lymphoid follicles that
form in response to antigenic activation of these cells.
Once initially activated in this local
environment, the lymphocyte will migrate
to the nearest draining lymph nodes,
which are the mesenteric lymph nodes.