How Viruses Mutate: Antigenic Drift and Antigenic Shift

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  • 0:05 The Different Ways…
  • 0:36 Antigenic Drift
  • 2:57 Antigenic Shift
  • 4:46 Viral Recombination
  • 5:46 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will discuss the major ways by which viruses, notably the flu virus, mutates. We will talk about recombination, reassortment, antigenic drift, antigenic shift, and more!

The Different Ways Viruses Mutate

If you've ever wondered why you need a flu shot every year, it's because viruses must constantly adapt to their environment. It's a normal process of evolution. One of the most famous examples of evolution due to viral mutation is the flu virus, or influenza. We'll focus in on the two major ways by which this virus can mutate, since it's the one that's most often in the news when it comes down to new viral mutations.

Antigenic Drift

The flu virus uses two main keys to get into, replicate within, and get out of the cells that make up your body. These keys are proteins on the surface of the influenza virus called hemagglutinin, or HA, and neuraminidase, or NA for short.

These proteins act as antigens, or molecules that activate your immune system's defenses in order to protect you. When your immune system comes across these foreign antigens, it produces antibodies that help to kill whatever possesses these antigens, which in this case is the flu virus.

However, gradual minor point mutations in the genes responsible for encoding the HA and NA proteins, called antigenic drift, may occur. These changes, again, are gradual and very small. However, they're large enough to cause your immune system to no longer recognize the HA and NA proteins on the new strain of influenza.

When your body is exposed to a particular strain of flu, which has a specific set of HA and NA proteins, it develops very distinct defense mechanisms against that unique flu virus, meaning your body stops only that specific key from fitting into any lock in your body, thereby rendering the virus virtually inconsequential.

Well, if all of a sudden a new flu virus with new HA and NA proteins waltzes in, your immune system won't recognize these new antigens very quickly because it was only trained to recognize the old set of keys.

These new HA and NA proteins are like a new type of key that can get into and open your lock. This new key will compromise your lock and will lead you to come down with the flu, while your body's adaptive immune system tries to build little proteins, called antibodies, against this new viral strain.

This is why you need that flu shot every year - so that you can boost your immune system against new viral strains of influenza!

Antigenic Shift

In very serious cases of viral mutations, something known as antigenic shift occurs. This is a sudden and major change in the surface antigens of a virus. Antigenic shift occurs when two different strains of influenza virus simultaneously infect the same cell in your body and undergo a process called genetic reassortment. This is a process whereby two viruses mix and match parts of their genome.

Since the genome is radically changed, so are the HA and NA proteins on the surface of the viruses that are coded for by this new genome. This means we are no longer talking about tiny little point mutation causing a slightly different key to be made. No, we're talking about a change so different that it's like using a retina scan to unlock a door as opposed to a key.

The change is so drastic that it can cause pandemics, or outbreaks of a disease, over large areas of land, such as multiple continents or even all over the world. That's because the virus is so new, barely anyone has any prior immunity to it, and a lot of people, therefore, end up getting sick.

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