Here IgM is initially produced but the B cells differentiate further into IgG producing cells. After about three weeks the primary immune response is turned off. However, during this initial period not only are Ab producing cells are made but also memory B cells are also formed. Consequently, when the same agent is encountered by the host again, the body recognises that its seen this before, and stimulates the memory cells to pump out Abs. This is called the secondary response.
Memory can last for a few weeks or alternatively can last for years.
Abs recognize antigenic determinants or epitopes . There are three types of effector immune response. There are literally millions of antigen specificities that the body can produce, and this is done genetically as the B cells mature. IgM - this is the largest Ab and the first one to be made in an antibody response. This is synthesized during the secondary immune response.