In this video we are going to review Specific Immunity and T cell activation
To begin let's review specific immunity. Specific immunity refers to the type of immunity which
mounts antigen-specific defense mechanisms. The specific response can take several days
to become protective. Specific immunity develops throughout ones
life time and involves B cells and T cell. In this video we will review the two types
of T cells; T helper and T cyotoxic cells. Each type of T cell plays a unique role in
the immune response and can be distinguished from the other by their surface markers. All
T cells have two distinct proteins on their surface,
The first one is the TCR -- the T cell Receptor and is the antigen receptor. Each T cell has
a unique TCR. The second protein is a cluster of differentiation marker, known as a CD marker.
The T cytotoxic cells have a CD8 marker and T helper cells have a CD4 marker.
Both surface markers are essential for the activation of T cells.
As we just saw T cells have TCRs on their surface, each recognizing a unique antigen.
Therefore, let's talk about antigens. There are two types of antigens, intracellular and
extracellular antigen. Intracellular or endogenous antigens, such as a virus or from a cancerous
cell are recognized by the cytotoxic T cells, T helper cells will recognize an epitope from
an extracellular or exogenous antigen such as the typical bacterial or protozoal infection.
The second surface marker on the T cell was the CD marker. The CD markers on T cells are
used to recognize MHC proteins on host cells. MHC molecules are self-proteins and are used
by APCs Antigen Presenting Cells to present antigens to the T cells
There are two classes of MHC molecules, MHC Class I and Class II
MHC Class I are expressed on nucleated cells and are used to present intracellular antigens
to cytotoxic T cells. MHC Class II are expressed on macrophages,
dendrite and B cells and are used to present extracellular antigens to helper T cells.
Let's take a second to review We have two types of T cells, T helper and
T cytotoxic. We have two types of antigens, extracellular
and intracellular and We have two types of MHC molecules, MHC Class
I and Class II. So how do they fit together, T helper cells
recognize extracellular antigens being presented with a MHC Class II molecule, whereas T cytotoxic
cells recognize intracellular antigens presented with an MHC Class I molecule.
So now let's put it all together and let's first talk about T helper cell activation.
T helper cells are used for extracellular antigens, to be activated they must interact
with an antigen presenting cell presenting antigen pieces with an MHC Class II molecule.
The helperT cell uses its TCR and the CD4 marker to recognize the antigen and MHC on
the APC. T helper cells require a second signal to be activated as well. The CD28 marker on
the T cell must recognize the CD80 on the APC. Once both signals or both recognitions
are achieved the Tcell can become active.
An active T cell is now able to bind to interleukin. Interleukin causes the T cell to proliferate
and differentiate. The T cell differentiates into memory T cell
and effector T cells. The memory cells go to the secondary lymphoid tissue to help with
future infections. The effector t cells, there are two, T helper 1 and T helper 2. The T
helper 2 cells are used to activate B cells in the humoral response this will be covered
in a separate video, whereas, T helper 1 cells increase the activity of immune cells such
as macrophages and help activate T cytotoxic cells.
So let's talk about T cytotoxic cell activation Cytotoxic T cells are used for intracellular
antigens to be activated they must interact with an antigen presenting cell using an MHC
Class I molecule to hold its piece of antigen. The cytotoxic T cell uses its TCR and CD8
marker to recognize the antigen and MHC on the APC.
The binding of the APC to the T cytotoxic cells initiates the T cytotoxic cell to display
interleukin receptors. The t cytotoxic cell can now be stimulated by Interleukin 2 which
was released by T helper 1 cells. Interleukin 2 will stimulate the T cytotoxic cell to undergo
clonal expansion producing both memory cells and active cytotoxic t cells
The memory t cells will go back to the secondary lymphoid tissue for subsequent infections.
The active T cell will be used to kill infected cells.
Let's review the killing mechanism of active cytotoxic T cells.
The active cytotoxic T cell will come in contact with an infected cell at the site of infection.
The cytotoxic T cell knows which cells are infected, because the infected cells present
antigen on their surface using an MHC Class I. Once the T cytotoxic cell finds the infected
cell it will bind to it. Through exocytosis the cytotoxic T cell releases perforin and
granzymes. Perforins are proteins used to poke a hole in the infected cell and granzymes
are degradation enzymes which break down any antigen pieces and cellular debris. The release
of perforin and granzymes initiates cell death in the infected cells. The death of the infected
cell is a controlled death so it doesn't lead to an infection of surrounding cells.
So in summary T helper cells are used in extracellular antigenic
responses, and require antigen to be presented with MHC Class II.
T cytotoxic cells are used in intracellular antigen invasions and are activated by APCs
presenting the antigen using the MHC Class I molecule.