Plasma cells differentiate from B cells once a B cell has interacted with its target antigen.
One the antigen binds to a B cell's surface receptors and the cell has interacted with a T helper cell, signaling takes place that triggers the B cell conversion to a Plasma cell. These cells migrate back to the peripheral lymphatic tissues such as the lymph nodes or spleen to undergoes their changes.
During differentiation, the cell will develop a large volume of Golgi apparatus to facilitate high volume antibody production. The cell will also lose its ability to respond to antigens as the cell surface receptors are downregulated.