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Antigenic Shift: Definition & Concept

Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

Antigens are distinguishing features that our immune systems use to recognize and remember invaders. Microbes can evade our immune defenses by changing their antigens. In this lesson, learn about antigenic shift with influenza virus as an example.

Definition

Antigens are the specific molecular structures that antibodies and other receptors in our immune systems recognize. Antigens and antibodies also form the basis of immunological memory, which is when the immune system responds faster and more powerfully to an infection that it has seen before.

From a microbe's point of view, it's much easier to infect a host, survive and replicate if the host's immune system doesn't recognize you. An antigenic shift is a sudden, drastic change in a microbe's antigens, and it can help microbes evade the immune system. Unfortunately for the host, this often makes microbes more pathogenic, or capable of causing disease.

It's important not to confuse antigenic shift with antigenic drift, which is a gradual and more minor change in a microbe's antigens over time.

Example: Influenza Viruses

Influenza Viruses

Influenza viruses are the viruses that cause the flu, one of the most common diseases in the developed world. When you get the flu, you get headaches, fever, chills, muscle aches and are just generally unhappy for a few days before you recover. But it can also be fatal, especially for very young and very old people.

The influenza virus is an enveloped virus that has an outer protein coat made of two spiky proteins called hemagglutinin, or H, and neuraminidase, or N. Inside the virus are eight segments of RNA that encode the virus's genome.

There are at least 15 types of H protein and at least nine types of N protein known. An influenza virus is named for which H and which N protein it expresses. For example, an H1N3 virus expresses H type 1 and N type 3.

Antigenic Shifts in Influenza

When two or more different influenza viruses infect the same cell, antigenic shifts can occur. Here's how.

A diagram of the antigenic shift that occurs when two influenza viruses infect the same cell

In this diagram, an H7N7 virus and a H3N2 virus have infected the same cell. They've each emptied their eight RNA segments into the cell's cytosol and begun to replicate them using the cell's own machinery.

When it's time to package the new viruses, each virus makes sure it gets one of each of the eight segments, but it doesn't care which original virus each one came from. This reassortment means that the new virus will have a random combination of RNA segments from both viruses that infected the cell.

In this example, the new virus got the RNA segments that encode H3 and N7, so this virus's surface looks completely different than the original viruses. The antigens have shifted.

Why Are Antigenic Shifts in Influenza a Big Deal?

The problem with antigenic shifts is that they can result in new viruses that are able to infect humans but that human immune systems have never seen before.

For example, normally, the influenza viruses that infect birds ('avian influenza viruses') are not able to infect humans. But if an avian and a human influenza virus infected the same host (for example, a pig in a large industrial farm), a new and dangerous virus could form. It could retain the infectivity of the human virus but gain new antigens from the avian virus, antigens that most humans' immune systems have never encountered before.

Such an antigenic shift could cause a major pandemic, and this is what people are worried about when they talk about swine flu and bird flu.

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