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According to Wikipedia:
A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe, its toxins or one of its surface proteins. The agent stimulates the body's immune system to recognize the agent as foreign, destroy it, and "remember" it, so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.
Question: How does the immune system ""remember"" a foreign agent introduced via a vaccine? And how does it learn how to deal with subsequent encounters?
Vaccines work by introducing an attenuated strain of the pathogen (or alternatively the antigens that are normally present on the pathogens surface) into the body, whereupon the body mounts an immune response. As this will (hopefully) be the first time that the body has encountered the antigens on the pseudo-pathogen's surface, the response is called the primary response.
This consists of two main divisions: the cell mediated pathway and the humoral pathway. In vaccination it is the humoral pathway that is important. This is where a division of white blood cells (B-Cells) produce antibodies that are complementary to the antigens on the pathogen surface, causing a negative effect to the pathogen (death, inability to reproduce, de-activation of toxins, etc.). However as each B-Cell produces a different antibody, there needs to be a mechanism to select the correct one:
- Antigen Presentation (technically part of the cell-mediated-response) - a phagocyte engulfs the pathogen and displays the pathogenic antigens on its own surface.
- Clonal Selection - B-Cells that are attracted to the invasion site attempt to bind their antibodies onto the pathogen's antigens. It takes time for this to occur successfully as you are essentially waiting for the correct mutation to happen.
- Clonal Expansion - Once a complementary antibody producing B-Cell has been found it is then activated with the help of a T-Helper cell. This causes it to divide rapidly whereupon these cloned specific B cells can secrete their antibodies which will cause detriment to the pathogen.
It's at this point that I can start to answer your specific question. The large clone of B cells will then sub-divide into two types. Plasma Cells remain in the blood and produce antibodies to fight the infection. The other type, much smaller in proportion, are called Memory Cells. These cells have a very long lifetime and move to lymph nodes across the body (including the spleen), where they remain dormant until the same pathogen is found again.
When this is the case, the memory cells are activated by T-helpers so that they can divide into massive numbers of plasma cells to fight the infection the second time. This secondary response is a much faster as the clonal selection stage does not have to wait for the right mutation - they are already waiting in the lymph nodes. The response is also much stronger as each memory cell can produce large numbers of plasma cells - i.e. you can start with multiple activated B-cells (as many as you have memory cells) rather than just the one that has mutated into a complementary shape in the primary immune response.
The aim of the vaccine is for the secondary response to be so quick that potentially life threatening symptoms do not occur; the body has time to find and store the correct antigen in a safe environment as the pathogen has been deactivated.
MBQ and Rory M have already given decent answers on the "hows" of how the vaccine memory is formed. Now, for some twists:
There are three (general) types of vaccine, all of which are meant to make your body "think" its being infected and provoke an immune response, while at the same time not causing active infection. They are:
- Live attenuated. These vaccines are made up of living pathogens that have, through a variety of methods, been rendered essentially harmless. Never the less, they are infecting you. It's a bit like getting into a shootout with people carrying Nerf guns. Good news is, the bullets are foam, but they're still shooting you. The advantages to these vaccines are that, generally speaking, they were easier to make, and because you are infected, you can shed the pathogen back into the environment, spreading the vaccine around a bit. The disadvantage? Sometimes the pathogens de-attenuate, and cause disease.
- Killed. Dead pathogen - your body responds to the presence of foreign antigens, develops an immune memory, and that's that. You gain safety, but lose the secondary spread of vaccinating attenuated pathogens.
- Virus-Like Particles. This is the sexy new class of vaccines, that is both dead and… not real. Synthetically produced, these vaccines (like the HPV vaccine) are both harmless and provoke an immune response, and can be customized - they can have antigens to different strains, etc. that you can't do just with attenuating or killing pathogens in the wild. They're awesome, and brilliant, and expensive as hell to develop and produce.
Once your body "learns" from the vaccine, how well it retains that memory also may vary. Some vaccines provoke a strong immune response, and the pathogen they target is genetically stable enough that you're pretty much set for life. Others, like the chicken pox vaccine, wane over time as your body isn't challenged by new exposures to the pathogen, and needs to be "reupped" now and again by giving you another dose of the vaccine, known as a booster. For some vaccines where your vaccine-triggered immunity may wane over time, we might not bother with a booster because you're now old enough that you're out of danger. The HPV vaccine comes to mind again - we're not sure you're immune for life, but if we can make sure you're immune until say, 65 or 70, it doesn't really matter, because it will take years for cervical cancer from an HPV infection to develop anyway.
Still others target diseases that mutate rapidly enough that your body's memory doesn't last long. The yearly flu vaccine is an example of this. A year after you get your vaccine, you body may "remember" the previous year's strain well enough to fight it off, but that's not the strain of influenza going around that year. This is also a problem for the hypothetical development of an HIV vaccine.
In short, some of the B-cells (antibody producing lymphocytes) specified to deal with this agent go dormant after the vaccine stimulation -- when the real danger comes, they can proliferate quickly and flood it with new antibodies, also alerting the rest of the immune system.
If you want a shorter version, but with some key terminology in there, here you go: When the body is exposed to an antigen (a substance that stimulates an immune response), antigen-presenting cells (e.g. macrophages) engulf them and present them on the surface of their cell membrane. Then, T cells bind to it and in short, they eventually get activated. Then activated T cells activate B cells, which then divide by mitosis to form a clone with plasma cells and memory cells. Remember, these plasma cells are specific to that antigen. The memory cells are the one that causes a quicker secondary immune response as it recognises the antigen when the body is exposed to it again and causes a greater number and faster production of antibodies.