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 (as commonly assumed among immunologists researching mammals) 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:
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.