B cells are lymphocytes, a type of white blood cell. Once the B cell is activated, it turns into a plasma cell, and starts producing antibodies. They are a vital part of the adaptive immune system. They have a protein on the B cell's outer surface known as a 'B cell receptor'. This allows a B cell to bind to a specific antigen.
The main functions of B cells are: to make antibodies against antigens, to perform the role of antigen-presenting cells (APCs), to develop into memory B cells after activation by antigen interaction.
Recently, a new, suppressive function of B cells has been discovered.
In mammals, immature B cells are formed in the bone marrow, hence their name.
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References
Other websites
B Cells and T Cells
B Cell Development and Generation of Lymphocyte Diversity
Interactive Animation of B Cell Maturation (requires Flash video software)