Once an infection occurs, to fight it out, B cell divides rapidly to form two types of cells, namely
Memory B cell and Plasma Cells
There are millions of B cells that are present in the lymph nodes that have never encountered an antigen before. These B cells are called naive B cells. On encounter with the antigen, the naive B cell divides rapidly to form Plasma B cells (or plasma cells) and Memory B cells. The Plasma B cells produce millions of antibodies to fight the infection. Memory B cells on the other hand, remember the antigen and help in providing a heightened immune response on encountering the antigen again.