How do my B-cells kill bacteria? Do they use knives and kill the bacteria? Use micro-guns and shoot them? I have trouble understanding how a 10 micron B-cell can kiill anything.
With extreme prejudice?
Good question, actually. I know that white blood cells are somewhat known of the enforcers of the immune system, but what does “kill” actually mean. Do they just neutralize things? Do they break them down into component parts? Take them out back and feed them to Wu’s pigs?
I’m sure someone will tell us.
Actually, by and large, B-cells are not responsible for killing bacteria. That task generally falls to another type of white cell called a neutrophil.
Neutrophils are loaded with “granules”, the contents of which include various enzymes and toxic chemicals such as peroxide. So, the neutrophil engulfs the bacteria and kills it chemically - with the enzymes and chemicals basically digesting the bacteria. BTW, this process of engulfing and “eating” the invader is called phagocytosis and the cells performing such a function (of which a neutrophil is one) is called a phagocyte.
OTOH, your lymphocytes are the virus killers. In brief, it is an orchestrated attack. Some lymphocytes may make antibodies against the virus thereby destroying it outright or signaling other lymphocytes to come and help. Some related cells, called macrophages, may engulf the germ and, as well, also signal other lymphocytes to the scene. If the problem is that one of your own cells is infected by a virus, and therefore needs to be destroyed, a special lymphocyte called a “killer T-cell” comes to the rescue.
How’s that for a start? I have to go, but this Wiki article on the immune system (which I haven’t read) may be a good source.
Oooh, oooh, I just learned the mechanics of this on Monday. When B cells can make antibodies against the bacteria (D. pertussis for example, after you’ve been vaccinated against pertussis), they do so because they’ve been given a present. Neutrophils and macrophages phagocytose the bacterial cell, and using a lysososme break down the bacterial cell. Cellular proteins are ubiquinated and chopped up into strings of roughly 7-15 amino acids in length. These amino acid strings are attached to a protein receptor called the MHC I that then carries the bacterial amino acids outside the neutrophil or macrophage. A B cell with a similar MHC protein transfers the bacterial amino acids to its interior, where it begins to make antibodies, each generation more specific for the bacterial amino acids (and thus the bacteria) than the last.
Otherwise, macrophages (and neutrophils, I believe) can present a protein called FasL (for Fas Ligand) on their surface. FasL binds to the Fas receptor on the target cell (in this case a damaged or malignant eukaryotic) cell, which then kick-starts a genetically programmed death cycle called apoptosis in the target cell. Sort of like the kiss of death for the damaged/malignant cell.
Vlad/Igor
B cells don’t directly kill invader organisms. They differentiate into Plasma cells and then secrete antibodies. These antibodies do many things when they attach. They may neutralize the bacteria or virus outright by blocking key receptors. They act like a signal which recuits phagocytes. And since antibodies are shaped like a Y with 2 receptors they can attach to two SEPERATE bacteria of viruses and cross link them together. Eventually this can create a large mass which makes them easy to destroy by phagocytes.
For larger organisms like parasites theres a special class of white blood cell that actually hovers around the parasite and lobs bags of lysogenic enzymes at the parasite like a catapult attacking a castle.
Uh Vlad, you may want to tone down the jargon for us mere mortals.
Translating some of the terms that Vlad used:
Phagocytose: Specific way that a cell engulfs and “eats” something else, in this case some foreign bacteria.
Lysosome: Part of a cell that digests and breaks down larger molecules and structures.
Ubiquitin: Protein that is used to mark another protein to be broken down. That target protein is said to be ubiquitinated.
Apoptosis: Cellular self-destruct sequence.
Thanks, lazybratsche. And I didn’t even get into the extrinsic vs. the intrinsic apoptotic pathways, either. 
My apologies, Rysto. This stuff is so cool to me, and I learned about the MHC and B cell response years ago.
Vlad/Igor
It should be pointed out the way plasma cells “figure out” what antigen to make antibodies for is by a process called VDJ recombination. As I understand it the cells edit their own DNA to do this and the ones that do a “Good job” recognizing and attacking a particular antigen get to become memory cells that produce more plasma cells later one.(So it’s kind of like your immune system evolves over time.) Again, my memory of this is kind of hazy at the moment.
Hey, Vlad/Igor, nice synopsis. I’ll just throw in a quick nitpick, as immunology texts are a bit behind. Most phagocytosis, and antigen presentation is done by a cell called the dendritic cell, and not macrophages as was previously taught. A really neat cell is the dendritic cell, by the way.
Hmm… if anyone has more details on the mechanism involved here, I’d love to hear them. I do remember that there are exceptionally high point mutation rates specifically at the sequences that code for variable parts of antibodies. I’ve heard this described as an evolutionary process. IIRC, it allows long-lived vertebrates to “keep up” in the evolutionary arms race with rapidly reproducing (and evolving) parasites.
Don’t forget about the complement system! When antibodies bind to a cell, they can become binding sites for something called “complement proteins” which are free in the blood. A little chain is formed, with different proteins adding on, until a “pore complex” is formed, which is essentially a way of punching a little hole in the cell. This messes up the cell’s inside/outside balance of salts, and the cell will die.
As for the B cells having the right antibodies, it’s pretty much a massive genetic shuffle, which produces almost every pattern imaginable. Each B cell has one pattern. Those that react against “self” antigens aren’t allowed to live, because then we’d be attacking ourselves all the time.
In general, the immune system is set up such that when an antigen presenting cell (dendritic cell, neutrophil, etc) finds something, it normally has a chance to meet tons of B cells, and basically call out to a roomful of them “hey guys - is this one yours?” Lymph nodes, and some specialized patches in the gut, are where the B cells hang out, waiting for something to do. Once a B cell recognizes its antigen and starts making antibodies, it also undergoes clonal expansion. That means it divides into a whole lot of copies of itself, so its clones can help it to make even more antibodies. Once the infection is under control, most cells will die, but some will stick around as memory cells, because if the bug hit you once, it may do it again, and it doesn’t hurt to be prepared.