Memory and naïve B cells normally exist in relatively small numbers.  As the body needs to be able to respond to a large number of potential pathogens, it maintains a pool of B cells with a wide range of specificities. Consequently, while there is almost always at least one B (naive or memory) cell capable of responding to any given epitope (of all that the immune system can react against), there are very few exact duplicates. However, when a single B cell encounters an antigen to which it can bind, it can proliferate very rapidly. Such a group of cells with identical specificity towards the epitope is known as a clone, and is derived from a common "mother" cell.  All the "daughter" B cells match the original "mother" cell in their epitope specificity, and they secrete antibodies with identical paratopes. These antibodies are monoclonal antibodies, since they derive from clones of the same parent cell. A polyclonal response is one in which clones of multiple B cells react to the same antigen.