Antibodies are produced when the body is exposed to a pathogen. Antibody production is a function of the specific immunity arm of the immune system.
During the first encounter with a pathogen, antibody production against that pathogen is a bit slow and not very vigorous. However, memory of that pathogen is kept in memory cell which are a type of lymphocyte.
On a second encounter with the same type of pathogen, the immune system specifically remembers the pathogen and the response this time round is faster, very vigorous, specific and very effective.
Vaccines follow this principle, making good use of immune memory.
