When we look
at the cells that are involved in immune reactions,
we can classify them into two different classes.
First of all, there are the lymphocytes, and
then there are the accessory cells or helper
cells, antigen presenting cells they’re often
referred to. The lymphocytes circulate
around the body in the blood, and they leave
the blood in various locations by means I’ll
describe throughout this lecture. And then
they can go through the tissues undergoing
surveillance on sensory duty to detect an antigen.
And these lymphocytes fall into two different
classes. B cells which differentiate in bone-marrow
and a bone-marrow derived, and the T cells
which although they derive from bone-marrow,
they need to go to the thymus gland where
they’re educated to detect and respond to
their antigens that they are trying to detect.
If you remove the thymus from an experimental
animal, they don’t develop T cells.
And for that reason, the T cells are thymus-dependent
on their education, T for thymus, T for
T cells. The B cells were allegedly described
in the bursa of Fabricius, which
is a little gland they discovered in chicken
embryos in the cloacal region. And they are
found to contain lymphocytes and removal of this
bursa of Fabricius experimentally result in no
antibodies being produced in the animal. So
they are called B for B cells, B for the
bursa of Fabricius. So that’s where the terms
B and T come from. And within the T cells, there
are two categories, helper T cells, cytolytic
T cells, and also other sorts of T cells.
The accessory cells are macrophages, dendritic
cells, and follicular dendritic cells, which
I’ll refer to throughout this lecture. When
we look at the B cells, the T cells, natural
killer cells, they are the key players. But as
I alerted to you in the previous slide, these
other four groups of cells play a very important
role. Usually, lymphoid tissues, organs such
as the lymph node and spleen, they are dominated
by a very, very elaborate spongy network that
consist of reticular cells. These reticular
cells secrete reticular fibres or collagen
type III, and they wrap around those fibres
and form this cobwebby spongy network right
throughout the organ. And they express little
flags on their surface to attract in all the
four cell types that you see listed in this
slide. The monocytes, macrophages, some of
the blood cells, some of the white blood cells
moving to these spaces, attracted by the
reticular cells. Reticular cells also hone in
dendritic cells. It’s important to understand that
Langerhans cells are dendritic cells, but
follicular dendritic cells are not.
They’re not derived from the stem cell. They live
in lymph nodes which we’ll see in a moment.
And the epithelioreticular cell is the word I’ll
describe later on that needs a little bit
of elaboration because again, epithelioreticular
cells are confined to the thymus. They are
the supporting cell in the thymus. They’re
not reticular cells. They don’t secrete
reticular fibres. They’re epithelial cells.
So perhaps, the name needs correction.
One way in which we find the histology of lymphoids
difficult in some respect is that, there are
many many different sorts of lymphocytes, and
they can be identified using certain markers
by immunohistochemical staining using monoclonal
antibodies. But if you don’t use these stains,
it’s very hard to differentiate different sorts
of cells. They express these CD molecules
called cluster of differentiation markers.
And various lymphocytes express certain sorts
of these CD molecules. Some cells express
particular CD molecules for their entire lifespan.
Others only express these CD molecules at
certain stage of their differentiation,
or when they’re activated. And based on those
three different criteria, immunologists
and medical laboratory scientists can
use histochemical staining, immunohistochemical
staining using monoclonal antibodies that I
mentioned a moment ago to actually identify
and differentiate these different sorts of