Video Transcript
The graph provided shows the
primary and secondary immune response in relation to antibodies produced. Which line represents exposure for
the second time to the original antigen?
Let’s start by looking at the time
it takes for primary and secondary immune responses. The primary response occurs after
the first exposure to an antigen. The response takes almost a week to
begin. During this lag period, B cells and
T cells go through clonal selection and clonal expansion, while the innate immune
system fights the infection using antigen-nonspecific methods. Clonal selection is where a B cell
or T cell is selected for cloning because it has receptors which recognize specific
antigens on the infecting pathogen.
The selected cell then proliferates
to produce many clones, which travel throughout the body and eliminate the
pathogen. During the clonal selection phase,
activated and memory immune cells are created. For example, memory B cells and
activated and differentiated B cells, so-called plasma cells, which produce
antibodies, are created. Memory T cells are also produced,
but we will keep things simple by concentrating on B cells here.
The primary immune response does
not reach its peak until more than two weeks have passed. Eventually, when the antigens are
cleared, the primary immune response is shut down. Activated cells are removed from
the system. And all that’s left behind after
about five weeks are memory cells. The secondary immune response
occurs as a result of a subsequent exposure to the same antigen. It is a much more rapid and
sustained response due to the action of memory immune cells. The presence of memory cells means
that there is no need for the time-consuming clonal selection process. B cells and T cells specific to the
antigen are already prepared to rapidly become activated. More antibodies are made by more
cells more quickly during the secondary response. These antibodies also persist in
the blood for a longer time after the infection has been cleared.
From this information, we can see
that during the secondary response, the body makes antibodies almost immediately
after exposure. And there are more cells making
more antibodies. The amount of antibodies will
therefore increase rapidly. On the graph, we can see that line
Y shows a rapid increase in antibodies — this occurs over less than one week — and
that the amount of antibodies remains high for a longer period of time. Lines X and Z show long periods of
time for the antibodies to increase. And the lines go almost flat after
around five weeks, which correlates with the description of the primary immune
response. Line Y represents the secondary
immune response the best. So the answer to our question is
Y.
