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So I get the concept that a vaccine is a weakened form of a virus so that the body can "learn" to fight it and make a person immune to that disease, but how exactly does this learning take place? What learns? The white blood cells? Do they have their own database or something and do they go from cell to cell informing them of what they learned? I read that they only last around 5 to 7 days so that means there's constantly new white blood cells that also need this information for all diseases ever encountered. How does this work? Thanks!

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  • $\begingroup$ See: quorum sensing and controversial study about Electromagnetic Signals from Bacterial DNA where it indicates a "wireless" version of intercellular communication which was found in bacterial communities, so it could be similar for white blood cells. $\endgroup$ – kenorb Mar 4 '15 at 13:02
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So while some kind of a biological database might have been an evolutionary option, the way we evolved is different from this. Rather than 'planning' what diseases could be recognized in the environment and recording this, our bone marrow constantly pumps out naive B and T cells that have a randomized receptor on them. The effect of this is that every new B and T cell that your bone marrow makes will recognize a unique, random protein sequence, and you will have billions of these cells floating around in your blood circulation at any given time. These cells however will only float around in circulation for a short period of time before dying and being replaced by other cells with new random sequence receptors.

When you get a vaccination, you are injecting in specific protein sequences from a specific pathogen. As these injected protein sequences circulate through your blood they will eventually bump into a B or T cell that has a receptor that recognizes them. When this happens, the B or T cell will now start to divide and give rise to long lasting memory cells that express the exact same receptor and will therefore recognize the exact same protein sequence again in the future. These cells will float around in your blood for most of your life and make you 'immune' to the recognized pathogen. When you get reinfected with the same pathogen or the pathogen to which you were vaccinated these memory cells will rapidly divide upon recognition of the pathogen and give rise to a large number of effector cells that eliminate the pathogen before it can 'make you sick'.

What is really cool about this is that because each T or B cell carries on it a receptor that recognizes a random protein sequence, you currently have in you B and T cells that would recognize a protein that does not even exist on Earth, but might exist on Mars or somewhere else in the Universe. This makes our immune system quite valuable because any protein that nature creates for all of time anywhere in the universe will still be recognizable by our immune systems.

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    $\begingroup$ Correct. The idea is that if a naive cell doesn't come into contact with and recognize a protein within a short while then it might not recognize a pathogen in your environment (or a protein sequence that even exists for that matter) and is eliminated so your blood doesn't just become a sludge full of too many cells. But when a naive cell does come into contact with and recognizes a protein it will produce a small population of memory cells that are very long lived so that they will be able to rapidly respond the next time you come into contact with the pathogen. $\endgroup$ – The Nightman Mar 3 '15 at 21:05
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    $\begingroup$ The B/T cell receptor is not only recognizing proteins, it is recognizing antigens in general. Most of these are proteins, but they don't have necessarily to be proteins. $\endgroup$ – Chris Mar 3 '15 at 21:11
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    $\begingroup$ There is also a subsystem to eliminate B cells that recognize "self" (which includes food). Obviously it's not good for the immune system to go after "self" molecules. And that's exactly what does happen in auto-immune diseases: the "identify self" subsystem glitches. $\endgroup$ – dmm Mar 3 '15 at 22:02
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    $\begingroup$ Right, I think a really good answer should cover clonal deletion and affinity maturation. The first explains why you aren't immune to "self" and the second is an important part element in making the polymorphic immune system even better. $\endgroup$ – Francis Davey Mar 3 '15 at 22:35
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    $\begingroup$ If this is the case, how do some vaccines last longer than others? $\endgroup$ – Eric Mar 4 '15 at 2:42

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