Video Transcript
Which of the following is not an
example of a cell a T lymphocyte would respond to in cell-mediated immunity? (A) A macrophage that has engulfed
a pathogen and displayed the antigens on its own cell surface. (B) A body cell that has been
infected by a virus and is presenting viral antigens on its cell surface
membrane. (C) Transplanted cells from other
individuals of the same species that have different antigens. Or (D) a red blood cell that has
begun to undergo changes in its plasma membrane as part of its programmed cell
death.
Let’s look at each of these
scenarios to determine which one would not stimulate a T lymphocyte response. A macrophage will recognize an
extracellular pathogen, such as a bacterium, engulf it, and display its antigens on
the cell surface membrane of the macrophage. It displays these antigens in major
histocompatibility complexes, or MHCs for short. It’s not just macrophages that have
MHCs. In fact, they are present on the
cell surface membrane of every cell that has a nucleus. Once a macrophage is displaying
antigens in its MHCs, these MHC–antigen complexes can be recognized by T
lymphocytes.
T lymphocytes have receptors on
their cell surface membrane known as T cell receptors. Each T lymphocyte has T cell
receptors, which recognize a different specific antigen. If a T lymphocyte encounters a
macrophage which is displaying its complementary antigen, the T cell receptor will
bind to the MHC–antigen complex of the macrophage. This will activate the T
lymphocyte, meaning it can go on to help activate other immune cells, which will
ultimately clear the extracellular pathogen from the body. We can therefore eliminate answer
option (A) because T lymphocytes do respond to macrophages displaying antigens.
Answer option (B) presents a
similar scenario to (A), except this time instead of a macrophage engulfing an
extracellular pathogen, we have an intracellular pathogen, a virus, which is already
inside a body cell. Remember, we said earlier that
every cell with a nucleus has MHCs. This is one of the reasons why. When a body cell becomes infected
with a virus, it processes the viral proteins and presents them as MHC–antigen
complexes on its cell surface membrane. T lymphocytes with complementary T
cell receptors then recognize these MHC–antigen complexes and become activated. This time, the activated T
lymphocytes will release cytotoxic molecules, which will kill the virally infected
body cell. We can therefore also eliminate
answer option (B).
The scenario described by answer
option (C) involves transplanted cells from an individual of the same species. An example of this is when someone
receives a stem cell transplant as treatment for a blood disorder such as
leukemia. It’s important to note that all
body cells that express MHCs present antigens. If these cells have not engulfed or
been infected by a pathogen, they present what are known as self-antigens.
A self-antigen is one which has
originated from a host cell rather than from a pathogen and is unique to a
particular individual. During their maturation, any cells
of the immune system which respond to self-antigens are destroyed. This is why our immune system does
not usually attack our own healthy cells. When someone receives transplanted
cells from another individual, although the cells are not harmful, they will have
antigens that are different to the body’s own self-antigens. This means they will be recognized
as non-self-antigens by complementary T cell receptors, and a T-lymphocyte-mediated
response will be mounted against them. Answer option (C) is therefore also
not the correct answer.
Finally, answer option (D) presents
a scenario in which a red blood cell is undergoing programmed cell death. You may recall that red blood cells
do not have nuclei, so they do not present antigens on MHCs. This means they cannot be
recognized by T lymphocytes. Red blood cells undergoing cell
death still have antigens on their cell surface membrane, but these will be
recognized by macrophages in the liver, spleen, and bone marrow. Therefore, the correct answer is
(D). A T lymphocyte would not respond to
a red blood cell that has begun to undergo changes in its plasma membrane as part of
its programmed cell death.
