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Question Video: Explaining How the Immune System Protects Us from Repeat Viral Infections Biology

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?

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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.

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