plasma cells are effector B cells. After a B cell gets stimulated by an antigen it can differentiate either into a memory B cell or into an antibody-producing effector B cell (plasma cell). The name of plasma cell comes from the older days of electron microscopy when people were just using TEMs to look at random samples, and they saw those cells – in addition to the red blood cells – in the samples derived from blood plasma.
at the original poster:
Do you know site specific recombination is used for?