The clonal selection theory proposes that when B cells encounter an antigen, they proliferate and differentiate into clones of plasma cells that secrete antibodies specific to that antigen. Some of these cells become memory cells that allow for a rapid secondary immune response upon re-exposure to the same antigen. The theory was developed in the 1950s by Niels Jerne and further expanded by Frank Macfarlane Burnet to explain how antibodies are produced and how immune memory develops. According to the theory, the immune system maintains a vast repertoire of B cells that can be selected and activated to respond to a diverse array of pathogens.