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.

Humoral immunity (the creation of antibodies that circulate in blood plasma and lymph) involves B cell activation.