Plasma cells. Animation showing plasma cells secreting antibodies (Y-shaped). Plasma cells, which are found in the blood and lymph, are mature B lymphocytes (white blood cells) that produce and secrete antibodies during an immune response. Antibodies bind to specific antigens, for instance viral proteins. In doing this they mark the antigen for destruction by phagocytes, white blood cells that ingest and destroy foreign bodies. Antibodies can also kill some pathogens directly and can neutralise toxins.