So how does the body "Remember" what diseases it can fight?

[Chronos]

I asked this question myself, in the year 4 BC:

How do antibodies work?

OK, so we all know the basic story of the immune system: A virus or other pathogen enters the body, the immune system develops an antibody to it and mass-produces it, the antibody neutralizes the invaders, and the immune system keeps the plans for the antibody in case the same pathogen invades again. But there are a lot of gaps in that basic story that need explaining. First of all, how does the immune system manage to develop molecules of the appropriate shape? Humans do that too, in drug d…