a cancer cell

The design of the immune system

One of the most beautiful images I have seen was the way that a certain type of immune cell generates its defenses.  The immune system is a beautiful defense mechanism – but what is it, and how does it work?

First, there are two main branches of the immune system.  Innate immunity, and adapative.  What does that mean?

Innate immunity is the system of defenses that automatically activates when anything weird happens.  It’s kind of like a house alarm that goes off if a window is opened or broken when the owners are out.

The innate response is rapid, occurring within seconds or minutes of something getting inside of you, perhaps if you got a cut accidentally.

Adaptive immunity is the developed response that eliminates invading pathogens.  Unlike the innate response, it has to be activated and developed, and can take more than a week to respond to something.

There were two great breakthroughs in the field of immunology that explained how the adaptive system is able to respond to almost anything whatsoever.  They both won Nobel Prizes.

There are more than a trillion possible things that your body can attack.  Each of these things is somewhat different from the others, yet your body can develop a receptor that is exactly tuned to each one of those trillion options.

This posed a great challenge.  After all, there are only about 30,000 genes in the entire human genome.  Even if every single gene coded for one target, that would still be far from adequate to protect your body.

The answer lies in recombination.  As my professor put it, it’s a lot like legoes.  With just a dozen or so lego shapes, you can build a startling variety of objects.

Your immune system breaks the development of its targeting into several blocks.  With 30 options for the first block, 30 for the second, 30 for the third, and 30 for the fourth, you already have 810,000 possibilities.  Yet to do so, you only need to have 120 blocks to build from.

By using the lego building approach, your body is able to develop a response to almost anything.  I was awestruck by its simplicity and elegance.

The second great breakthrough in immunology

With the development of vaccines, we were able to prevent and eliminate some of the worst diseases like smallpox and polio in the industrialized world.  Vaccines alone are responsible for a significant increase in additional years of life expectancy.

But how do they work?

Vaccines inject into your body a tiny particle.  This particle may or may not be combined with other things to make the response stronger.  Your body sees the weird particle, knows that it is unusual, and takes a week or two to develop an adaptive immune response to it.

But a) how does this prevent the disease from coming back later?

b) How does your body generate the immune response so quickly?

The answer lies in memory and clonal selection and expansion.

It is a pretty simple idea.  Basically, your body keeps billions of immune cells waiting at certain points over your body, particularly in those called lymph nodes.  These stations monitor the fluids in your body for the presence of foreign substances.

So imagine a billion b cells kind of waiting inside this sampling station.  They all have different receptor targeting and react to a slightly different substance.  When one of them recognizes its activation signal, it springs into action.

Once activated, it goes to a special growing area and starts dividing rapidly.  One cell becomes two, then four.

2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096,

And after just a few rounds of cell division, you have many, many copies of that one cell, all of which are targeted to the substance that activated the cell to begin with.

These cells then go and do what they do to eliminate the invader.

Once it is eliminated, most of them die.  They simply aren’t needed anymore once there is no more of their target.  But some of them stay dormant; they are stored and remain waiting for that moment.

When their nemesis comes back into the body, the response is different.  Instead of a delay, the memory cells quickly recognize that their target is back – and they rapidly grow to mount an immune response to it.

This development of memory and immunity means that once you defeat a disease, you typically don’t have to worry about it again for the rest of your life.  It also makes vaccines work, because a vaccine helps your body generate the response and the memory cells to respond quickly to further provocation.