B cells are lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response). The abbreviation "B" stands for the bursa of Fabricius which is an organ unique to birds, where B cells mature. It does not stand for bone marrow, where B cells are produced in all other vertebrates.

The human body makes hundreds of different types of B cells, and each type has a unique receptor protein on its membrane that will bind to one particular antigen; at any one time in the human body millions of B cells are circulating in the blood and lymph, but are not producing antibodies. 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. Cell activation can be gauged using the ELISPOT technique, which can determine the percentage of B cells that secrete any particular antibody.

B cells are characterised immunohistochemically by the presence of CD20 on the cell membrane.

Susumu Tonegawa won the 1987 Nobel Prize in Physiology or Medicine for demonstrating how B cells create the enormous diversity of antibodies from only a few genes.