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If all of the DNA is being used, then how would the cell be able to store new information about pathogens in DNA? It's like a full hard drive that can't hold any more. So does the cell just compress the information about the pathogens and then just adjust the existing information about pathogens?

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All the body does is produce a ton of cells that can recognize single antigens. Each time you encounter pathogens, some of these naïve cells contact antigen that can activate them. The activated clone replicates itself, forming effector cells that can deal with pathogens.

As the infection is cleared and the population of effector cells begins to contract (i.e. many die off or become anergic), some of them undergo changes which alter their migratory pattern and longevity. Some can stay in the tissue that was originally infected as tissue-resident memory cells. Some home back to the lymph node and hang there as central memory cells. The lineage is affected largely by chemical signals being produced by the tissue and other immune cells in the vicinity.

Upon re-encounter of the same pathogen, if it has some of the same antigenic determinants as the original infection, these memory cells are already known to be specific to them. Encountering the same antigen will cause these memory cells to undergo another round of clonal expansion, producing more effector cells and additional memory cells so the process can repeat. These events can be limited by telomere length or inhibitory molecules, as a caveat.

The key understanding is that the immunologic memory is actually stored in the repertoire of long-lived memory cells, and in part results from recombination events at the T or B-cell receptor locus during their maturation before they're released as naïve cells.

See: Abbas (2014). Cellular and Molecular Immunology, 8th ED., many different chapters.

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  • $\begingroup$ There is also 'intrinsic' and 'innate' immunity. Immune cells can recognize common patterns on pathogens, such as lipopolysaccharides, double stranded RNA and flagella and respond to these shared patterns. These common responses are more ancient. 'Adaptive' immunity is relatively new. $\endgroup$ – Artem Mar 7 '17 at 18:44
  • $\begingroup$ That makes a lot of sense. Like I said to the other person, I'm just taking AP Biology at school, so I'm not very knowledgable of this field. So if you get an infection on your foot, and your lymph node with those memory cells is around your neck, will it send those memory cells to the foot, or does the lymph node just protect its immediate surroundings? Also, you mentioned the textbook. Do you recommend that book for further research at my level of knowledge, or should I start out with less advanced material? $\endgroup$ – Garrett Smith Mar 7 '17 at 21:32
  • $\begingroup$ There are pathogen sensing cells called Antigen Presenting Cells which nibble on bacteria and proteins in different environments, these cells then migrate back to lymph nodes where memory cells are 'present' the antigens they picked up. If there's a match then you initiate the so called 'adaptive immune response'. $\endgroup$ – Artem Mar 7 '17 at 21:45
  • $\begingroup$ So in that case, would the Antigen Presenting Cells only make it to the local lymph nodes? In that case, memory cells in the neck would not be helpful to an infection in the foot. $\endgroup$ – Garrett Smith Mar 7 '17 at 21:58
  • $\begingroup$ Central memory T cells are considered a circulating subset. They home to lymph nodes temporally (time-dependent) through the expression of the receptors CD62L and CCR7. There's actually a neat paper in immunity about how T cells circulate on a circadian clock of sorts, and so they come out of the lymph nodes and circulate every now and then. This basically means your central memory T cells aren't always in the neck, and it's how a DC in the draining lymph node in the foot can find a naïve or memory cell to prime. $\endgroup$ – CKM Mar 8 '17 at 16:23
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You're asking for a course in immunology, which is far too long to include here.You have several misconceptions. Short and oversimplified answer, the information is not stored in one cell, but in trillions, each with a single piece of pathogen information. Within each cell, the information is stored in DNA, which in these particular cell types can be modified. Your hard drive analogy, like most such analogies, is completely misleading and you should abandon it. There are several introductions to adaptive immunity online; you might find one (such as here, here, or here) helpful.

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  • $\begingroup$ Sorry, I'm just taking AP Biology, and it was a topic that we're learning about. I definitely want to get more into it, so I'll probably try to find some resources to learn more. Do you have any recommendations aside from waiting until college? $\endgroup$ – Garrett Smith Mar 7 '17 at 21:22
  • $\begingroup$ The IMGT writeup is probably a good starting point -- I think if you're taking AP Bio you should be able to follow it pretty well. You might jump to the "Mounting an Immune Response" section for a quick overview. The Wikipedia article on the Adaptive immune system goes into more detail; you might want to follow some of the links in the introduction. $\endgroup$ – iayork Mar 8 '17 at 13:29

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