Video Transcript
The time between being infected
with a pathogen and the body producing an immune response can be referred to as a
lag phase. Which of the following correctly
compares the lag phases of primary and secondary immune responses? (A) The lag phase of the primary
immune response is usually much shorter than the lag phase of the secondary immune
response. (B) The lag phase of the secondary
immune response is usually much shorter than the lag phase of the primary immune
response. (C) The lag phases of the primary
and secondary immune responses are highly similar.
This question is asking us to
compare the lag phase of the primary and secondary immune response. Let’s remove the answer choices so
we have more room to work with.
Lymphocytes, like T cells and B
cells, are part of the adaptive immune system. These cells recognize antigens on
the surface of pathogens and mount an immune response to clear the infection. B cells can become activated in
response to recognizing their antigen and divide and differentiate into plasma
cells. Plasma cells can release antibodies
to combat the pathogen. Here is a graph that shows the
amount of antibodies produced over time after first being exposed to a pathogen. This is called the primary immune
response.
As stated in the question, the time
between exposure to a pathogen and the immune response is the lag phase. The primary immune response occurs
when an antigen is recognized for the very first time. The lag phase for the primary
immune response is long because the immune system doesn’t immediately recognize the
antigen. Once a lymphocyte like a B cell
develops that can recognize this particular antigen, this cell divides and
differentiates into plasma cells and memory B cells. These plasma cells can produce
antibodies to target the pathogen. This is what causes the antibody
levels to rise. Once the infection is cleared,
these plasma cells are deactivated and die. This causes the antibody levels to
fall.
Now let’s see what happens when a
person is exposed a second time to the same pathogen. Because there’s memory B cells that
were created from the first exposure, these cells can respond immediately to a
second exposure. These can divide and differentiate
into plasma cells, which can quickly secrete a large amount of antibodies to fight
this infection. We can see that this secondary
immune response happens much more quickly and produces much more antibody. As a result, the lag phase during
the secondary immune response is usually much shorter than the lag phase during the
primary immune response.
Therefore, the correct answer is
(B). The lag phase of the secondary
immune response is usually much shorter than the lag phase of the primary immune
response.
