Video Transcript
The figure shows a body cell that
has been infected with a virus under attack by a cell of the immune system. What is X?
The figure presents a cytotoxic T
cell interacting with a virally infected host cell. When host cells become infected
with a virus, they digest and process it before presenting unique viral proteins on
their cell surface membrane. These unique viral proteins are
called antigens, and they are presented on special molecules called major
histocompatibility complexes, or MHCs for short. All T cells, including cytotoxic T
cells, possess T cell receptors, like the one we can see here. Each T cell has T cell receptors
which recognize a different specific antigen.
When a cytotoxic T cell, like the
one in the diagram, encounters a cell presenting its complementary antigen on an
MHC, it becomes activated. The activated cytotoxic T cell has
a few mechanisms for destroying the infected host cell, one of which is to release a
chemical called perforin. As the name suggests, perforin acts
to perforate the infected host cell by forming pores in its cell surface
membrane. These pores allow cell contents to
leak out of the cell, as well as enzymes to enter and digest internal cell
proteins. As a consequence, the infected host
cell dies. Because viruses rely on host cells
and their proteins in order to replicate and spread, this is an effective mechanism
for removing the pathogen from the body. We have therefore determined that
the molecule labeled X on the figure is perforin.
