A few days before I started reading Immune I accidentally cut my finger while dicing an onion, but after uttering a few choice expletives and fishing a band-aid out of a drawer I went back to making supper without giving it another thought. According to Philipp Dettmer, there was a lot going on in those few drops of blood. Each one contained about 250,000,000 red blood cells, 400,000 immune cells, 15,000,000 platelets, and 13,000,000,000,000 antibodies.
The more you know about the immune system, the more amazing it gets: layer after layer of sophisticated defenses, some of them quick reacting and general purpose, to immediately start fighting intruders while the main defenses gear up. Others are slower but more specific, taking several days to analyze the threat and build specific weapons to counter it. This is why you are often sick for a couple of days before you start feeling better, because it takes time for the body to marshal and deploy its full array of defenses.
Most of us have heard about macrophages and Killer T-cells, and we have a vague idea that antibodies are floating around in our blood doing something to help keep us healthy. These are only a few of the different types of immune cells, which trigger and interact with each other in subtle and complex ways.
It can be argued that the most important part of your car is not its engine but its brakes, because while moving is nice, being able to stop is critical. Similarly, the immune system’s most important feature is its restraint: able to effectively repel invaders but not so hair-trigger sensitive that it indiscriminately attacks and destroys normal, healthy cells. For this reason your body’s most powerful defenses must receive a separate confirmation signal before they can be activated, sort of like two-factor authentication for computer systems.
The first line of defense is our skin, and the book has a chapter examining its various layers and how they create an effective barrier to keep bacteria and other threats out. A single millimeter separates the dead skin cells on top from the living ones below, but in that distance multiple stages play out, as the cells are generated and then compress, spread out, connect to each other, forming a tight chemical bond that ensures there are no gaps for invaders to penetrate.
There are multiple levels of threats that the body must be able to respond to. If you get a cut, bacteria swarm in, because bacteria are everywhere, in fantastic numbers. The body is tuned to recognize Self from Other, and since bacteria are clearly Other they are instantly attacked, while other systems work to close the wound and repair the blood vessels damaged by the cut.
Viruses represent a more sophisticated level of threat, because they do their work inside the cells, invisible to many of the body’s defenses. To counter this the cell’s surface is studded with proteins which let the immune system “see” inside, and will order the cell to kill itself if anything is amiss. The cell does not just die, which might release the viruses inside, but slowly folds in on itself, creating pockets that trap the viruses until a macrophage can come along and consume them. Of course, viruses evolve just like everything else, and in their case the generative processes are very sloppy, so that each generation can have new mutations, which means that purely by random evolutionary chance some of them will be better able to defeat the body’s defenses, such as by eliminating the surface proteins that let the immune system examine cell contents.
Worst of all are cancers, because they are the body’s own cells. Even so, the immune system does a good job recognizing and destroying cancer cells, and is doubtless doing so right now somewhere in your own body. However, some cancers are more insidious and harder to detect than others, and as we get older our body’s defenses get weaker and less effective.
Finally, in addition to fighting off threats, the body must remember what it has battled in the past, so it can respond immediately in the future. This, of course, is the fundamental rationale for immunizations, so get those shots. Measles is particularly dangerous, because it specifically targets the cells which hold your immune system’s memories, so not only can it kill you directly – and it is one of the most contagious of all diseases – but it can leave you open to reinfection by pathogens your younger, stronger, healthier body defeated decades ago, and now you have no immunity to them.
The author of this book had to walk a fine line between burying the reader in scientific detail and describing things in such a simplified manner that important topics got glossed over. In general, he does a good job covering all the bases without making the reader’s head spin with biochemical complexity. To do so, however, he adopts an approach which I found jarring. After correctly emphasizing that the immune system’s components are just biological machines doing what evolution assigned them to do, and that they have no agency of their own, he nevertheless describes them in extravagantly anthropomorphic terms, such as “Suddenly a jolt of energy shoots through the Macrophage’s bloated body. In a heartbeat, its spirit comes back and it feels fresh again. But there is something else: A hot, white anger. The Macrophage knows what it needs to do: Kill bacteria, right now!” (p. 99)
Nevertheless, despite this authorial quirk, the book is successful as a general, nontechnical introduction to the human immune system. It does a good job explaining the wondrously sophisticated systems that keep us alive, and its bibliography points to the reader to where to look for more technical information. Life has been a 3.8 billion year evolutionary arms race since the first cells appeared, and in that time a marvel of powerful, ingenious defenses has been created, carefully balanced between aggression and restraint. The fact that it sometimes goes awry and causes autoimmune diseases is unfortunate, but the real wonder is not that those things happen occasionally, but that they don’t happen to all of us all the time.