After getting fully vaccinated for HBV why does the antiHBs titer last for so long? When a newborn child is fully vaccinated with HBV and let's assume he's responsive, his antiHBs titers will be high. But since the antigenic stimulus is withdrawn, shouldn't the titre fall to nill in some time? Rather, the titers stay significant for around 40 more years. (>10 IU/L is significant) So, how are there circulating antibodies for so long if there is no more antigenic stimulus? I understand that memory cells have large half lives but what is producing the antibodies is the plasma cells which have very short half lives. _So how do the antibodies persist for so long?_

There is such a thing as memory plasma cells, discovered only relatively recently. Here is a review from 2010 discussing them, here is another review from 2012, and here is a pre-publication manuscript of an upcoming review in _Leukemia_ (I apologize if any are behind paywalls, let me know if you can't access any of them).