Thanks very much, and all feedback is welcome!
This 45-sec. animation is a re-cap of the antigenic shift mechanism by which a brand new strain of influenza can be created. Please follow this link:
Thanks very much, and all feedback is welcome!
The mini pop quiz question from last post was:
What is the difference between antigenic SHIFT and antigenic DRIFT?
The answer is: TIME.
Yes, I gave you a trick question! The fact of the matter is, both mechanisms effect a genetic mutation that leads to a phenotypic change in the resulting flu virus. In the graphic, the viruses labeled #1 are significantly different from the viruses labeled #2. Each genetic change in each mechanism is responsible for an outward change, a new characteristic that makes the progeny virus unique. This is GOOD for the virus, but BAD for our immune system, for if you recall, B-cells are responsible for cataloging previously encountered viruses (see lower half of graphic) and other pathogens. Our immune system works efficiently because of this catalog; the cells that survey our bloodstreams for bad pathogens can more quickly recognize those that have already been encountered and filed away into memory. And the resulting response is so much faster and stronger than for the first encounter.
Both the shifting and the drifting mechanisms can produce entirely “new” virus strains (#2) that are unrecognized by our immune system. It is like encountering an entirely new pathogen, and has to start from scratch in identifying it as “bad.”
Getting back to our pop quiz, while both mechanisms produce the same outcome--a strain of virus that is “new” to our immune system--the genetic mutations produced in shifting occur on a different time scale than in drifting. Antigenic drift may not initially result in a phenotypic shift. In essence, the child virus may be practically identical to that of the parent. But over time, with the accumulation of many mutations over MANY generations of flu (occurs by errors in genetic copying), an entirely new strain of virus can be produced. In antigenic shift, where two viruses that infect the same host may in fact mix up their genetic material, the resulting new viruses may contain some segments from one viral strain and some from the other viral strain. Thus, in ONE generation, a new, unrecognizable, phenotypic viral strain can be created.
Obviously, there are more differences between antigenic shift and drift other than time. I was feeling especially clever when writing this post. Thanks for indulging me!
As spring has finally sprung, colds and the flu should be fast becoming a distant memory. However, with the prolonged cold/hot/cold weather we’ve been experiencing in Washington, DC, as the cherry blossoms bloomed so too, has a new late-season flu virus. This has prompted a new wave of patients (including my husband) who, despite having had the flu shot back in November, have now become victims of this new strain of flu.
We have most likely just witnessed the phenomenon of antigenic drift. A new, progeny flu particle that has undergone a slight genetic mutation from its parent, is slightly different from that parent. And as time goes on replication occurs very rapidly. Mistakes in copying genetic material from one generation to the next occur, thus producing slight variations in each successive genome. What does this mean? As with a human parent and child, these flu particles are very closely related; therefore, they may also be similar enough to each other such that if our immune system is already primed to the parent, it will likewise be primed to the child.
This is called cross-protection.
Now, take a look at the graphic. I’m taking some liberties with this as technically I should be continuing out along one branch, to represent successive generations of parent to child viral progeny. But this is a prettier presentation!
Starting from the right-side and moving counter-clockwise, we see that as point mutations within the genome occur, if the flu particles are closer together on the tree, there is still cross-protection from our immune system afforded between these two flu particles. This is because they resemble each other so closely that our immune system may “mistake” the mutated version for the original version, and thus, our immune system quickly remembers it as bad, and the response mounted is thus swift and strong.
But as we look at flu particles farther away from each other on the tree, for instance across from each other on the tree, we see that cross-protection no longer occurs between the two distantly-related viruses. The mutations that have occurred have altered the virus so much that our immune system no longer recognizes it for the original that it knows. We have to then start from scratch with cells coming upon it, packaging it and presenting it to another cell to recognize it as bad, and then triggering our immune system to mount an adequate response.
This accumulation in point mutations make two flu viruses quite different. A primed immune system to one flu virus may also protect from the second, slightly altered flu virus. Over time, as more and more mutations occur, the resulting flu resembles the original less and less.
Mini pop quiz time! What is the difference between antigenic SHIFT and antigenic DRIFT? Answer, next post.
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