Video Transcript
The varicella-zoster virus is the
virus that causes chicken pox. If a person is infected with this
virus when they are younger, they are highly likely not to be affected by chicken
pox again when they are older. Why is this? (A) The person’s barrier defenses
will be stronger after the first infection and prevent future pathogen entry. (B) Memory T cells are produced
after the first infection, and these initiate a rapid immune response if a person is
infected a second time. (C) Cytotoxic T cells will
eradicate the virus before it can reach the body cells when infected for a second
time. Or (D) this is incorrect; if they
catch chicken pox once, they are highly likely to be affected by it again.
Let’s remove the answer options for
now and explore what happens when a person experiences repeat exposure to a virus,
such as the varicella-zoster virus.
A virus is an intracellular
pathogen. This means it has to get inside
host cells in order to reproduce and establish an infection. All the cells of the human body
with a nucleus express major histocompatibility complex, or MHC, proteins. So cells that are infected with a
virus are able to present viral antigens on their cell surface membrane. When a person is first infected
with a virus, their nonspecific immune response will be activated. This predominantly fights the viral
infection with immune cells called natural killer cells, which recognize antigen
bound to MHC and release chemicals to destroy infected host cells.
However, the nonspecific response
is rarely sufficient to completely remove the virus from the body, so the specific
immune response must step in as the next line of defense. You may recall that the specific
immune response mainly involves T cells and B cells, both of which require an
antigen to be presented to them in order to become activated. The most important cell type in a
specific immune response against a virus is the cytotoxic T cell.
A cytotoxic T cell with receptors
that are complementary to the viral antigens will bind to the MHC–antigen complex on
the host cell and become activated. The T cell will then undergo clonal
expansion, where it is cloned many times. Many of these clones will
differentiate into effector T cells, which will travel around the body killing
infected cells and eliminating the virus. But some clones will differentiate
into a class of cells known as memory T cells, which will remain in circulation
throughout the body long after the infection has been cleared.
If a person becomes infected with
the same virus later on in their life, these memory T cells will quickly become
activated as cytotoxic T cells and fight the infection much more rapidly and
powerfully than the first time. In fact, this secondary immune
response is so efficient. We often don’t experience any
symptoms and therefore might not even know we have been infected again.
We have therefore determined that
the correct answer is (B). If a person is infected with the
varicella-zoster virus when they are younger, they are highly likely not to be
affected by it again when they are older because memory T cells are produced after
the first infection. And these initiate a rapid immune
response if the person is infected a second time.
