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I've just recently learned about the body's defense mechanism about how lymphocytes from memory cells that are specific to a pathogen after being infected by a pathogen for the first time, but then I saw that vaccinating people with cowpox can give people immunity to smallpox, which is similar to cowpox. Why can this happen? Shouldn't the memory cells formed in this case be specific to only cowpox?

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  • $\begingroup$ cowpox and smallpox are nearly the same disease, much like how the bat strain of covid and the human strain of covid are very related diseases. journals.plos.org/plosone/article?id=10.1371/… $\endgroup$
    – John
    Jun 18, 2023 at 16:45
  • $\begingroup$ As suggested by John, I think it is worth interrogating what you mean when you say that cowpox is "similar" to smallpox. What is similar between them? What is different between them? What would make the memory cells be "specific" to one or the other? Possibly, look into the history of smallpox vaccination: ncbi.nlm.nih.gov/pmc/articles/PMC9709930 $\endgroup$ Jun 18, 2023 at 20:55

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I think the key point here is to note that pathogens are assembled from multiple subassemblies, which are themselves composed of various proteins. It's these subassemblies of proteins which B cells develop antibodies for, in which case we call that subassembly an "antigen." The antibodies then attach to those antigens, which tags the pathogen for removal by other cells, destruction by proteins (in the case of membrane attack complexes), or otherwise prevents that part of the pathogen from doing whatever it would normally. There are also T cells, which have their own kind of antigen receptor (T-cell receptors). Have fun looking into those! You might check out "The Red Queen: Sex and the Evolution of Human Nature," by Matt Ridley, which is all about the Red Queen hypothesis, and dives into the major histocompatibility complex.

So, in the case of smallpox and cowpox, they share common antigens. That's what's being referred to in papers like "A comparison of the antigens present on the surface of virus released artificially from chick cells infected with vaccinia virus, and cowpox virus and its white pock mutant." Smallpox and cowpox are both orthopoxviruses, which you can learn more about from ViralZone's description.

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