How Viruses Steal Your Cells
They are not cells, and they have none of the internal structures that cells use to go about the business of life, which is, generally, to make more life. No, viruses are just genetic material looking for a free ride – looking to hijack a host cell and make its machinery do the virus’s bidding.
Rule for Viral Success #1:
Mutation, Mutation, Mutation
With so little to call their own, how have these biological pirates survived for so long? The answer lies in two traits that give viruses superb evolutionary advantages: superfast reproduction and genetic mutations.
Viruses live to reproduce. Although they must do this within host cells, once inside, viruses replicate with enough abandon to shame a rabbit. They quickly reprogram the machinery that cells use to copy their own DNA and use it to spit out copy after copy of themselves.
Genetic mutations add insult to injury. With so much reproduction going on, viruses can mutate almost as fast as they propagate. And massive mutation means that each new generation of viral invaders stands a good chance of gaining some new survival or targeting advantage.
Rule for Viral Success #2:
Pick a Likely Victim
Viruses invade all kinds of cells – plant cells, animal cells, fungi, even bacteria. Yet each virus tends to have a very specific M.O. Which cells look like likely victims to a virus depends on the unique proteins found on the virus’s protein coat and the protein receptors found on the poor target cell.
Some viruses recognize the general receptors that occur on many different kinds of cells. The virus for rabies, for example, can invade so many different kinds of cells that it can span species, infecting rodents, dogs, and humans.
Other viruses are more restricted and can invade only specific kinds of cells. The common cold virus, for example, can invade only the cells lining the human upper respiratory tract. It’s a picky thief.
Rule for Viral Success #3:
Make It an Inside Job
Viral entry mechanisms are as diverse as viruses themselves, which is why viruses often elude treatment. Some enter a target cell by binding to a specific receptor and passing through the host cell membrane to the cell interior. Others don’t need to enter the cell, but simply attach to the surface and use a needle-like structure to inject their DNA right in.
Once viral genes are inside, the virus begins its cycle of replication. It exploits the host cell’s supplies and machinery, forcing it to copy viral genes and synthesize more viral protein coats. Then, these two components come together to form copies of the virus that emerge from the host cell.
Sometimes they “bud” off the cell, like bubbles on top of a simmering stew. At other, more violent times, copies simply fill the cell until it can hold no more. It explodes, releasing its viral hoard into the surrounding area.
Either way, the viral progeny go on to infect new cells – and the cycle starts again. Disease symptoms can and do result from this cellular damage. Most often, though, the sickness you feel is the result of your immune system’s response to the foreign invader. And make no mistake, it will respond.
Rule for Viral Success #4:
Avoid the Cops
Your immune system’s first-responders act like beat cops on patrol 24/7. If they see anything amiss while walking the body’s beat, they make arrests. One kind of cellular cop, the phagocytes, will engulf strange viruses and digest them. Another kind, natural killer cells, recognizes suspect changes on the surface of infected cells and releases chemicals to disintegrate both virus and cell alike.
After spotting the infection, your body can launch a more specific and intensive attack. Proteins called antibodies surround, bind to, and neutralize viruses and other invaders in your bloodstream. Killer T cells mercilessly destroy infected cells and halt systemic infection. Both help your body remember the infection and mount a faster response to the same invader next time.
Still other players merit mention. When a cell does get infected with a virus, sometimes it manages to secrete small proteins called interferons that serve to warn neighboring cells of an imminent viral invasion. These “Paul Revere” proteins work by encouraging neighboring cells to synthesize proteins that can interfere with viral replication.
