Well, if we assume that your immune system takes a look at something that showed up in your nasal membranes once, and never reproduced, and said "that looks lik

Well, if we assume that your immune system takes a look at something that showed up in your nasal membranes once, and never reproduced, and said "that looks like a big problem we should learn how to make antibodies against". It is entirely possible that your immune system is smart enough to take a look at that and say "that looks like a bunch of inert junk", and does nothing, because it doesn't need to.