The spleen contains many immature B cells, which, because of the large amount of blood passing through the spleen, become exposed to new antigens and differentiate into functional cells. Each mature B cell can be stimulated by a specific antigen entering the body. After a B cell comes in contact with an antigen, it changes into a plasma cell or a memory cell.
Plasma cells produce antibodies that destroy the invading cell.
Memory cells are available to produce antibodies quickly if the same antigen reappears. All B cells are antigen specific and respond to only one foreign protein. The antigen-antibody reaction is part of the specific immune response. It is the basis for vaccination and plays a major role in defense against infection from common organisms. Taber's Cyclopedic Medical Dictionary, Edition 18.