What is the mechanism responsible for long-lasting detectable antibody titers?

As I understand, when naive B cell encounters antigen matching its receptors and is activated by a T helper cell, it can either differentiate into 4 plasma cells, produce a lot of antibodies and commit apoptosis, or into 2 memory cells waiting to be activated the next time they encounter the antigen.

On the other hand, IgG can be detected for a long time after infection. As antibodies decay, there must be long-term production of IgGs to keep their level constant. What is the mechanism behind that if there is no following infections?