In humans, B cells are released into the blood stream from the bone marrow as fully functioning adult cells. B cells are of course antibody producing cells. Most B cells situate themselves in immune organs such as the spleen and wait for their activation. Like T cells each B cell is only able to respond to one specific antigen and each B cell responds to a different antigen. The B cells have antibody on their cell surface which acts as the receptors for antigen.
B cells can receive stimulation in two ways.
Typically most antigens are first processed by antigen presenting cells and then presented to the B cells at the same time as presentation to Th cells. Thus normally receives two stimulating events. First its antibody receptors bind to the antigen and second it receives cytokine stimulation from stimulated Th cells. A few antigens can directly activate B cells. These antigens are free floating in solution and have not been processed in any way.