B cells (abbreviated "B" for the bursa of Fabricius, an organ uniqe to birds where B cells mature; the "B" does not stand for bone marrow, where they are created in all other vertebrates) are lymphocytes that play a large role in the primary immune response.

There are two types of B cells:

• Plasma B cells secrete antibodies which effect the destruction of antigens by binding to them and making them easier targets for phagocytes.
• Memory B cells are formed specific to the antigen(s) encountered during the primary immune response; able to live for a long time, these cells can respond quickly upon second exposure to the antigen for which they are specific.