According to a couple of sources I’ve heard, the human body is born with a library of all the possible antibodies it can ever produce. When a new and unrecognized infection shows up, the antigens get sent to this library and are compared against all the dormant antibodies stored there. When a match is found, the B cell carrying these antibodies is instructed to replicate itself, and its clones gets sent out into the blood and lymph systems to do battle.
However, a couple of years ago I took an introductory college-level course in molecular biology, and the prof presented a very different model of how new antibodies are produced.
In this other model, there is no library of dormant antibodies for the new antigen to get looked up in. Instead, some mechanism in the lymph node starts manufacturing one new antibody after another, trying random permutation after random permutation, until one of these “experiments” sticks to the antigen. Then the DNA sequence that produced this successful antibody becomes part of the legion of B cells that are created.
So … which is it? Or do we know? I tried reading Wikipedia’s articles about antibodies, the immune system, the adaptive immune system, and immunological memory, but they didn’t appear to describe the mechanism for this antibody-discovery phase.
No, yes, both, and sort of. We are definitely NOT born with all possible antibodies - the number of possible permutations is astronomical. We ARE born with a repertoire of a whole lot of different B cells, each of which produces a unique antibody, so we have a good solid library to start with. In addition, we keep making new B cells throughout our lives, and each one randomizes its antibody gene, so the library is constantly changing. This means that when we’re invaded by some nasty bug, there’s a pretty good chance that something in our library will react to it.
Now, when an antibody binds an antigen, the B cell that makes that particular antibody activates and starts dividing, making a whole clonal population that makes the same antibody. Almost. As part of this clonal expansion, there’s a process called “somatic hypermutation”. As these new B cells are produced, they start slightly changing their antibody genes. From there, natural selection takes over. Each B cell reproduces at a rate proportional to how well the antibody reacts to the antigen, so over time, the cells making better versions of the antibody outreproduce those making bad versions, and the body as a whole is making better and better versions. Bad versions die out. So pretty quickly, your body has narrowed in and made one or a few really really good antibodies that get the job done.
When the infection is eradicated, some proportion of those B cells making really really good antibodies stick around as memory cells to protect you from future infection.
BTW, is there a special name for the B cells that form that “library”, as distinct from the other B cells in your body? Like “repertoire B cells” or “dormant B cells” or “phi-beta-kappa B cells” (for the sorority they belong to)?
Plasma cells is the term for the B cells that become a specialized little antibody factories. They are fairly restricted in terms of what type of antibody you can get, but you’ll get lots of antibodies finely tuned to a particular antigen.
There are also whole classes of various types of memory cells, and that is a a current research area, as different types of infections activate different types of memory cells. Some are identified by the combinations of molecules they produce or what gene they have activated that other groups do not have activated.
The Genius Within: Discovering the Intelligence of Every Living Thing, By Dr. Frank Vertosick, has a detailed discussion of how this works (and a fairly novel way of thinking about it; he draws analogies to computer networking and regards the immune system as essentially “intelligent.”)
Vertosick explicitly says that the cells that best bind to the pathogen are the ones that are “allowed” to reproduce, essentially using the engine of natural selection to rapidly solve the puzzle the pathogen presents.
He also says individual insects are born with all the immunities they will ever have; insects reproduce rapidly enough that they use subsequent generations as their natural selection engine to develop resistance. SO your thought in the OP essentially describes how insects work, but not mammals.
