Video Transcript
The figure shows changes in
antibody concentrations in the blood after exposure to antigens A and B. How does the peak concentration of
antibodies in the secondary response approximately compare to the peak primary
response for antigen A? (A) There are two times more
antibodies produced in the secondary response. (B) There are three times more
antibodies produced in the secondary response. (C) There are 100 times more
antibodies produced in the secondary response. (D) There are 1,000 times more
antibodies produced in the secondary response. Or (E) there are 10,000 times more
antibodies produced in the secondary response.
This question asks us about primary
and secondary immune responses which are involved in the adaptive immune system. This is also sometimes called the
specific immune response since the response is specific to the antigen
presented. Let’s get rid of the options for
now so that we have room to go over some of the key facts about the adaptive immune
processes.
We have said that the response is
specific to the antigen presented, so what is an antigen? An antigen is a substance that when
recognized as non-self by the immune system will trigger an immune response. It could be a toxin, pollen, or
molecules on the surface of pathogens or other foreign cells. Now, let’s look at how antigens
trigger the adaptive immune system to respond.
The adaptive immune system develops
over time as a result of exposure to different antigens. It is made up of two main groups of
cells, the T cells and the B cells. T cells are then broken down into
three main groups: helper T cells, cytotoxic T cells, and suppressor T cells. Cytotoxic T cells kill infected
body cells. And suppressor T cells deactivate
the cells after the infection has been successfully fought off. Helper T cells assist the other T
cells and B cells to become activated. The activation of B cells leads to
their differentiation into plasma cells which secrete the antibodies. These bind to the antigens to block
infection and to flag them up to the roaming phagocytes. Phagocytes are cells which engulf
pathogens and destroy them using digestive enzymes. You can easily spot them in a blood
smear because of their large, lobed nucleus.
We have looked at the cells of the
adaptive immune response. But the question is asking about
the production of antibodies during the primary and secondary response. So let’s look at what these
are.
The primary response occurs after
the first exposure to an antigen. The B and T cells have receptors on
their cell surface membrane, which are important for a process called
activation. When a receptor on a helper T cell
encounters a specific antigen, it becomes activated. After activation, the helper T
cells proliferate, multiply in number, and secrete signaling proteins, such as
cytokines, to help activate cytotoxic T cells and B cells. B cells can be activated by binding
to an antigen alone. But usually, they require
interaction with an activated helper T cell and stimulation by the cytokines the
helper T cell has released.
Once activated, the B cells
proliferate and then differentiate into either plasma B cells or memory B cells. The plasma B cells produce the
antibodies against the antigen. And as their numbers increase, the
antibody level in the blood increases, as shown on the graph.
The question is asking about
antibodies against antigen A. And here, they are peaking at the
end of week two at 10 arbitrary units. The memory B and T cells are very
important for the secondary immune response, which occurs if the body is
reinfected. The memory cells sit in the bone
marrow and lymph nodes, as well as being carried in the blood. Their job is to monitor the blood
for the antigen and then respond if they do come across it. If they do encounter the antigen
again, they will be activated as before and increase in number. This is a much more rapid and
efficient response than the primary response, since the T and B cells are already
primed to recognize and destroy the antigen.
If we return to the graph and look
at the antibodies against antigen A, the one which triggered the primary response,
you can see that the peak level of antibodies is 1,000 arbitrary units, much higher
than in the primary response, which is why it is so effective. Now that we have reviewed these key
points, let’s return to our question and use the numbers we have read off the graph
to come up with the correct answer.
We have worked out that the peak
level of antigen A antibodies in the blood during the primary response is 10
arbitrary units. And the peak level during the
secondary response is 1,000. If we look more closely at our
graph, we can see that this graph is not labeled like your typical graph. That is because it is logarithmic,
which means that the values on the 𝑦-axis are multiples of 10 instead of increasing
at steady intervals. Scientists sometimes use these
types of graphs when data is obtained over very large ranges of values since it
makes it easier to visualize them.
To find how many more times the
number of antibodies in the secondary immune response compares to the primary immune
response, we can divide 1,000 by 10, which gives us 100. This means that the secondary
immune response produces 100 times more antibodies than the primary immune
response.
Let’s now bring back the answer
options so we can decide which one is correct. We can see that the correct answer
is (C). The way that the peak concentration
of antibodies in the secondary response approximately compares to the peak primary
response for antigen A is that there are 100 times more antibodies produced in the
secondary immune response.
