A B cell is a type of cell in the human immune system that is derived from bone marrow. With help from another type of cell, called a "helper cell," the B cell can transform into a plasma cell and secrete antibodies that latch onto a virus or bacteria. The antibody then acts as a marker that tells the immune system that the invading cell needs to be destroyed.
B cells are part of what's called the humoral immune response because it involves substances in our bodily fluids, which were historically known as humors. The humoral immune response fights infections through secreted antibodies. That process is not the same as cell-mediated immunity, in which such cells as T cells kill infected cells or help immune cells like macrophages kill pathogens.
Preliminary research suggests that the cancer drug Rituxan (rituximatab), which lowers the level of B cells, may be an effective treatment for a subgroup of people with chronic fatigue syndrome. Researchers say this could indicate autoimmunity in at least some cases of chronic fatigue syndrome.