Each B cell contains multiple copies of one kind of antibody as a surface receptor for antigen. The entire population of B cells has the ability to specifically bind to millions of different antigens. When the antibody on the surface of a B cell binds to an antigen, the cell can be stimulated to undergo proliferation and differentiation. This process is called clonal selection. The cells produced make the same antibody, but become memory cells and plasma cells.
Memory cells insure that subsequent infections by the pathogen receive a more rapid response.
Plasma cells secrete large amounts of the antigen-specific antibody. T helper cells, part of the cellular system, are usually required for the clonal selection of B cells. Antibody secreted by plasma cells forms complexes with free pathogens and their toxic products. The complexes can inactivate pathogens and stimulate other innate systems including phagocytic cells and complement to eliminate the danger from our extracellular fluids. Some pathogens can escape antibody detection by infecting cells.