B cells look a lot like T cells in their size and shape except they never have granules present in their cytoplasm. The "B" stands for Bursa of Fabricious, the organ only found in some birds where B cells go to mature. B cells were first identified in birds and found associated with this organ. Conveniently B can also stand for bone marrow where B cells mature in non-bird species like ourselves. In humans, B cells are released into the blood stream from the bone marrow as fully functioning adult cells. They constitute between 5 and 15% of all mature lymphocytes.

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. The B cell Thus normally receives two stimulating events. First its antibody receptors bind to the antigen and second it receives cytokine stimulation from stimulated Th cells. This form of activation is called "T dependent".

A few antigens can directly activate B cells. These antigens are free floating in solution and have not been processed in any way. They are usually very large molecules that can bind to numerous antibody receptors on a B cell simultaneously. This multiple binding seems to trigger the B cell into a response without any stimulation from Th cells. An activated B cell responds in similar fashion to Th or Tc cells. The few cells responding to an antigen proliferate into large clone populations. Most cells become "plasma cells". These are large cells with a lot of cytoplasm and are entirely given over to producing antibodies. These antibodies are released into the blood stream to be carried to the site of infection. The B cells themselves do not migrate to an infection site, it would be a waste of time and energy and they would get in the way. Antibodies are the long distance smart bombs of the immune system. They diffuse throughout the entire body and only bind to the particular antigen they were made for. A few of the clone cells that result from antigen activation become memory cells. Like the T memory cells they store the information about previous encounters with pathogens and if there is any subsequent challenge these cells will proliferate faster and produce antibody more quickly compared to virgin B cells.