Tip:
Highlight text to annotate it
X
In an adaptive immune response antigens must be processed and presented to T lymphocytes.
This is accomplished by cells known as APCs or antigen presenting cells. The process in
which an antigen is displayed is dependent upon the type of antigen. Is it exogenous
or endogenous?
In exogenous or extracellular antigens, the antigen is internalized by the cell through
endocytosis. This will lead to the formation of a phagosome. As in phagocytosis, the phagosome
will fuse with a lysosome. This leads to the destruction of the antigen. Once the antigen
is digested into smaller pieces it will fuse with another vesicle containing a self-marker
known as a MHC Class 2 molecule. In order to display the antigen it must be loaded onto
the MHC Class 2 molecule. Next the cell will move the vesicle containing the MHC:Antigen
complex to its surface and the membrane of the vesicle fuses with the cell membrane.
The MHC:antigen piece is displayed on the surface for presentation. Now the cell is
an APC with a MHC Class2:Antigen complex and ready to activate T helper cells.. Remember,
only macrophages, dendritic cells and B cells use the MHC Class 2 to display antigens.
In endogenous or intracellular antigens, such as in cancer or a viral attacks the antigen
originates within the cell. Therefore, the cell will use polypeptide fragments within
its cytoplasm to get the antigen. To make the antigen an appropriate size the cell will
use proteasomes to degrade the proteins into peptides. The peptides are transported to
the ER. In the ER the cell loads a piece of the peptide onto a MHC Class 1 molecule for
presentation. Ultimately the MHC:antigen molecule is released as a vesicle from the Golgi apparatus.
The vesicle moves to the surface of the cell where it fuses with the cell membrane. The
MHC:antigen complex is displayed on the surface for presentation. All nucleated cells are
able to process endogenous antigen in this manner, acting as an APC using an MHC Class
1 molecule for antigen display.