Video Transcript
Complete the sentence. We repeatedly get some viral
diseases such as influenza because the proteins on the outside of the virus blank
rapidly, constantly producing new blank that are not recognized by memory cells in
the circulation. (A) Denature, antigens; (B) mutate,
antigens; (C) mutate, antibodies; (D) hydrolyze, antigens; or (E) denature,
antibodies.
This question is asking about how
we can get some viral diseases, such as influenza, again and again, whilst with
others we will only get them once. To answer this question, let’s
review the key facts about the adaptive immune response and how viruses trigger
it.
The adaptive, also known as
specific, immune system develops over your lifetime. It involves the T and B cells,
which work together to fight infections. They respond to the presence of
antigens. Antigens are substances, usually
containing proteins, that when recognized as nonself by the immune system will
trigger an immune response. These include substances such as
toxins, pollen, and molecules on the surface of pathogens, for example, proteins on
the outside of viruses. So let’s now look at what happens
when a virus invades our body.
When the B and T cells come across
a virus for the first time, a primary response is initiated. This involves activation of the B
and T cells and then proliferation, rapid increase in number, to produce a
large-enough population of cells to fight the infection successfully. Helper T cells assist the B cells
to respond to the virus, and cytotoxic T cells kill infected body cells to try to
prevent further multiplication of the virus. The B cells produce specific
antibodies to the antigen on the outside of the virus. These bind to and neutralize the
virus. They also help the phagocytes to
engulf the viruses and break them up with hydrolytic enzymes. It takes between four and seven
days for the antibodies to start being made. And so it takes about two weeks to
shut down the viral attack.
Once this has been done, the active
B and T cells are deactivated. However, some cells remain as
inactive memory B and T cells. These cells will recognize the
specific antigen if it reenters the body. If the antigen does return, then
these memory cells are activated as before and multiply rapidly to destroy the
antigen. This secondary response is much
quicker and more effective than the first, so you don’t even feel ill. This is how vaccines work to
protect people from diseases.
Since adaptive immunity relies on
antigens being easily recognizable, the adaptive immune response will fail if these
antigens change too much. This is why diseases such as
influenza can reinfect a person that has already had it because the antigens found
on the proteins of the influenza virus can change, known as antigenic drift. Since proteins result from genetic
information, this is ultimately caused by mutations in the viral genome. When these antigens change, new
strains of the virus are formed. This is why the components of the
annual influenza vaccine are reviewed and updated as needed to keep up with evolving
influenza viruses.
So why do you not catch measles
more than once? The measles virus does not mutate
quickly, so the proteins and antigens it produces are fairly constant. This allows the body’s secondary
immune response to recognize the virus properly and protect us from subsequent
infections.
Now that we have reviewed the key
facts about the adaptive immune responses and how some viruses have evolved to evade
them, we can return to our question and narrow down our answer options. We now know that the B cells
produce antibodies to neutralize the antigens, which means that antibodies are not
part of a virus. We can therefore rule out options
(C) and (E).
If a protein is denatured, its
structure is broken down, and when a phagocyte uses hydrolytic enzymes, it is
breaking down the virus. So these processes are not involved
in making new proteins. We can therefore also rule out
options (A) and (D).
This leaves us with the correct
answer, option (B). We repeatedly get some viral
diseases such as influenza because the proteins on the outside of the virus mutate
rapidly, constantly producing new antigens that are not recognized by memory cells
in the circulation.
