Microorganisms and Health [3]: Lymphocytes (A Level Biology)

Uploaded by freeeschool on 02.09.2012

A Level Biology: Microorganisms and Health 3 – Lymphocytes
Hi! Welcome to my third video on the series about Microorganisms and Health. Today, we
are going to be looking at further immune responses.
So we are going to be looking today at what happens after a phagocyte encountered a microorganism
or a pathogen and then the steps taken by your other leukocytes in order to eradicate
that infection.
What happens after a microorganism has been ingested by a phagocyte is that remnants of
that digested pathogen found within the phagocyte are actually the antigens that haven’t been
digested. Remember, the antigens are the surface proteins found on the surface of most cells.
These are things that your white blood cells can use to determine whether something is
foreign or whether it is yours.
What happens is those antigens are then presented by the phagocyte on its surface, represented
on this part of the diagram here. This green bit represents the antigen that was on the
pathogen. Then it shows that to a relevant T cell. Now, T cells are what are known as
lymphocytes. They’re another type of white blood cell. They are referred to as T cells
because they are formed in the thymus. In this case, the antigen is being shown to a
CD4 T cell and CD4 simply means Cluster Designation 4. It is a name that we give to a certain
type of T cell.
These CD4 cells are the ones actually affected by the HIV virus. They act very much like
a general in the army and they actually coordinate the response. They bring in other relevant
If you look at this section here, it says about the Antigen-specific T cell receptor.
Now, antigens have a very, very specific shape. So, an antigen on the surface of particular
bacteria will be different to the antigens on the surface of a virus. There are very,
very specific receptors found of the surfaces of these cells. It takes some time to locate
these specific receptors and indeed, the specific CD4 cell.
Once the correct CD4 T-cell has been found, that then releases a series of chemicals called
as cytokines and you can think of cytokines like a communication method between immune
cells. From there, the CD 4 cells bring in several different types of cells, but two
main ones. T Cytotoxic cells and the name would suggest cytotoxic and so a toxic cell
use toxins in order to break down pathogens. Also this cell here, very, very importantly,
B cells and B cells are the ones that produce the antibodies that can really clear up the
infection very, very quickly. B cells get their name from the fact that they are produced
in the bone marrow.
Of course, a bit like the CD4 T-cells, the CD8 Cytotoxic cells and the B cells are very,
very specific. They send out a very particular cytokines to attract the correct type of B
cell and the correct type of other T cells and that’s all down to their specific shape.
For instance, different B cells will produce different-shaped antibodies. In this case,
what you want is an antibody that will actually fit the original pathogen.
The same is true with the CD8 cell. You want to bring in a CD8 cell that has a very, very
specific shape and therefore, it can act on the pathogen. It is this whole process that
takes some time to go from getting infected to being out to produce the correct antibody
because you need to find the specific cells that can actually tackle the problem.
In summary, what first happens when you get infected by pathogen within your bloodstream
is phagocytes encounter that, they break down and they digest the pathogen but they don’t
break down the antigen found on the surface of that pathogen. They then present that to
a type of lymphocyte called a T cell. The T cells are like generals in the army and
they coordinate a response because they bring in correct type of other white blood cells
or other lymphocytes.
The T cells can then bring other types of T cells, namely: Cytotoxic T cell and the
B cell and these B cells are the ones that produce the antibodies.
In the next video, we will be looking at going on to how antibodies work and exactly the
next stages in the immune responses.
[end of audio – 04:29] A Level Biology: Microorganisms and Health
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