An organism that is infected with an extracellular pathogen
produces an increased amount of varying antibodies
via T-helper cell-activated B cells in
the germinal center of lymphoid tissues.
If the same organism is later reinfected with the same pathogen,
the macroorganism will respond with a much stronger response,
producing antibodies specific to that
antigen in a shorter amount of time.
What is the term for this process, that allows B-cells
to produce antibodies specific to that antigen?
Answer choice (A) -
Answer choice (B) - Avidity
Answer choice (C) -
T-cell positive selection
Answer choice (D) -
and answer choice (E) -
T-cell negative selection
Now take a moment to go through the answers by
yourself before we come to a conclusion together.
Okay let’s discuss the question.
The first thing we need to do is
determine the subject of this question.
And in this case, we are dealing
with an immunology question.
We have infections of a cell, we have T-helper cells, we have
B-cells, we have lumphoid tissues, we're talking about antibodies.
This is classically immunology.
Now this is a 2-step question.
We first need to be able to identify the process that’s
allowing the B-cells to produce specific antibodies
and then we have to know
the name of this process.
And the stem is absolutely required in this
case because it's going to the detailed process
that we need to be able to interpret
to properly answer the question.
So let’s walk through
this question together.
Well for step 1, we need to determine what immunological
step is being described in the process in the question stem.
Now the vignette is referring to a stronger
immunological response in the case of a reinfection.
Now antibodies produced by B-cells are more specific
at reinfection than when you had it at first exposure.
Now the process is describing the increase of
antibody specificity after first antigen exposure.
So we have to be able
to identify that first.
Second, we have to know what that process is called.
Now the increase of antibody specificity actually
occurs at various stages during B-cell maturation.
And that’s very elegantly
depicted in our image.
Now if you look at our image, it’s showing the
maturation of B-cells in a germinal center.
So if you look at where it says dark zone B-cell, what you
see they’re after is what’s called clonal expansion.
Really this is monoclonal expansion of the B-cell and then
what happens is the B-cells undergo somatic hypermutation.
Now this triggers gene recombination and the mutations that occur in
this process actually increase the affinity for the given antigen.
And you see that here in the image.
You have the dark zone B-cells, it then undergoes
monoclonal expansion then we have somatic hypermutation.
And as the mutations are occuring
in the somatic hypermutation,
what we’re actually doing is increasing
antigen affinity during this process.
Now recall that the term affinity is antibody-antigen
binding strength, so we’re actually increasing this.
Now when we are undergoing somatic hypermutation,
we’re actually creating multiple clones of B-cells
and the next step is actually the
selection of the highest affinity ones,
Now you actually have apoptosis of the lower
affinity ones which you also see on the image.
Now somatic hypermutation and selection,
when these two things are combined
is a process known as affinity maturation, which is
actually the answer to this question - answer choice (A).
Now selected B-cells progress either to
become plasma cells for antibody production
or memory B-cells to further accelerate
immune reaction at the time of reinfection.
So the image here we see
is extremely high-yield.
We’re seeing B-cells undergoing monoclonal
expansion, then we’re seeing somatic hypermutation,
and then that process we’re actually
increasing our antigen affinity.
And then as they go on to become plasma cells or memory B-cells, we have an ability to produce specific antigen
and also have memory B-cells
available for reinfection later.
So now let’s discuss some high-yield
facts regarding this question.
The first thing we need to discuss is the answer
choice in this case, which is affinity maturation.
Now production of antibodies with highest single-site binding
strength to certain antigens occurs with affinity maturation.
Now where this occurs is actually in
the germinal center of lymph nodes.
Which is what we've discussed earlier as
being what we are seeing in our image
which is the maturation of B-cells occuring
in the germinal center of lymph nodes.
If you actually look at what the word 'affinity maturation' means,
it comprises of both somatic hypermutation and the selection
of the highest affinity antibodies which is what
the process is described in the question stem.
Now let’s discuss a little bit
more about somatic hypermutation.
Genetic recombinations and mutations
that create a wide variety of antibodies
when the organism first encounters a foreign
antigen is what occurs in somatic hypermutation.
Now this actually provides a range of B-cells and
antibodies for selection which is a great process.
We get a lot of antibodies to pick from from our
B-cells in this somatic hypermutation process.
And this overall process is
called affinity maturation.
Now there is a term also discussed here in
the answer choices which is called avidity.
Now this refers to the overall binding strength
across multiple affinities or binding sites.
Now generally, we use this term when describing
antibody-antigen interactions across mutiple binding sites.
For example, IgM has low affinity but high avidity
due to its 10-low affinity binding sites.