Alan Stahler: Head-start
You don’t have to go into the woods to meet up with something that wants to eat you – we are surrounded by thingies that see us as dinner. Cut your finger, and bacteria are right there, ready to munch away. And then there are viruses, which see us, not as food, but as factories – factories to make more viruses.
Battling these bugs is the job of our immune system. Immune cells recognize these bugs, and latch onto them, much as a piece in a jig-saw puzzle latches onto another. But there are gazillions of different bacteria and viruses. How can our immune system have so many different jig-saw puzzle pieces, ready to pounce?
The instructions we need to keep ourselves alive are written in a couple-dozen threads of DNA – super-thin, but totaling six feet long. With the exception of our red blood cells, pretty much every cell in our body carries these instructions.
This information is precious.
Each of us begins life as a single cell in mom’s belly. This cell divides to make two; two divide to make four; on and on, cells divide to make the gazillions of cells of which our bodies are built.
Before each division, the DNA must be copied, so that each “daughter” cell can receive a complete set of instructions. Copies are made with utmost care, but – even so – mistakes are made. An important part of the copying process is proofreading – using a sort of “spell-check”, each copy is compared with the original. Mistakes are cut out and corrected. The resulting copies are astonishingly accurate …
… but not perfect. A small number of errors slip through. Such mistakes in DNA are mutations. Most mutations – the vast majority – do not do us any good. Quite often, a cell with mutated DNA simply dies. If it lives, the mutation may go unnoticed … or it may cause a problem, such as hemophilia, or cancer.
The body works hard to keep the mutation rate low. And yet, in one part of the body, the proofreading system – the “spell-check” – is shut down. Here, we make mutations like mad, resulting in hundreds of millions of mutant cells. We are all – you and I – thus mutants.
These mutants are generated by our immune system. To make the hundreds of millions of different jig-saw puzzle pieces – to be ready for germs it’s never yet seen – the immune system shuts down spell-check. The resulting mutations create hundreds of millions of different jig-saw puzzle pieces, making a bet that at least one of these puzzle-pieces will match a puzzle-piece on a bacterium, on a virus … will allow the immune cell to latch on to that germ … and kill it.
All well and good. But what if one … or a hundred … or a thousand … of those mutant puzzle-pieces happen to match a puzzle piece attached to one of our own body cells? Our immune system would attack us. Autoimmunity is why some folks cannot eat gluten: For those with celiac disease, gluten triggers the immune system to attack its own body.
Immune cells that target our own cells must be destroyed.
Between each of our fingers is a bit of webbing. Back some hundreds of millions of years ago – back when our ancestors were fish – that webbing stretched completely between “fingers.” Fish don’t need hands … fish need fins.
In the hundreds of millions of years since we’ve parted ways with the fish, we have not lost the habit of growing webbing between our fingers. When we’re babes in mom’s belly, our fingers are webbed – completely. Before birth, we don’t grow hands – we grow fins. To turn those fins into hands – hands with fingers – we’ve got to get rid of that webbing.
The webbing between fingers is composed of cells that are very much alive. To get rid of those cells, our DNA sends a message to them. It tells the web cells to commit suicide.
Among people, suicide is a tragedy – a tragedy for the individual, a tragedy for family and friends. But among body cells, suicide is commonplace … and essential.
Now and then, we perform a little accidental surgery on ourselves – we cut ourselves, scrape our knees. Cells die, and leave a mess to be cleaned up – bits and pieces of cells to be mopped up and carted away, a wound to be healed.
When cells commit suicide, they do not simply die. Rather – in a carefully-programmed sequence – the cells take themselves apart … slowly and neatly. Rather than a mess, the result is a collection of cell-parts that other cells in the body can re-use – to repair themselves, to grow, to make new cells.
The babe’s finger-webs disappear … with nary a scar to be seen.
As our immune system churns out gazillions of jig-saw puzzle pieces … it compares them with the puzzle pieces on the cells in our bodies. Should a puzzle-piece match one of our body’s puzzle-pieces, the immune cell bearing that puzzle-piece gets a signal to commit suicide.
OK – We’ve got gazillions of immune cells, each bearing a different jig-saw puzzle-piece, ready and waiting to attack an invading germ. But when germs get into us, the first thing they do is multiply … we soon host gazillions of them. To repel this army of germs, we’ll need gazillions of immune cells, all carrying that same puzzle piece that fits puzzle-piece on the germ. Our immune stem, busy as it’s been, churning out cells bearing gazillions of different puzzle-pieces, now needs to replicate the one cell with the appropriate puzzle-piece. This takes time … days, even a week.
All while the germs are multiplying like mad.
We can get a head-start, though, if we’ve already seen that germ before. If that bacterium, that virus infected us some time in the past, our immune system remembers, and gets a HUGE head-start in churning out defenders.
The French word for cow is “vache” (“vosh”) Hundreds of years ago, people in the French countryside noticed that milkmaids, exposed to cowpox (a very minor disease in humans), were immune to smallpox. After experiments showed it would work, doctors proceeded to inoculate everyone with cowpox, beginning the process that has erased smallpox from the planet.
From the word “vache,” we get the word “vaccine” – something that plants a memory in the immune system, giving the immune system a head-start when the bug arrives.
Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at firstname.lastname@example.org.
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