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B cells and T cells are the main types of lymphocytes. B cells work chiefly by secreting antibodies into the body’s fluids. Antibodies ambush antigens circulating the bloodstream. They are powerless, however, to penetrate cells. The job of attacking target cells - either cells that have been infected by viruses or cells that have been distorted by cancer - is left to T cells or other immune cells. Each B cell is programmed to make one specific antibody. For example, one B cell will make an antibody that blocks a virus that causes the common cold, while another produces an antibody that attacks a bacterium that causes pneumonia. When a B cell encounters its triggering antigen, it gives rise to many large cells known as plasma cells. Every plasma cell is essentially a factory for producing an antibody. Each of the plasma cells descended from a given B cell manufactures millions of identical antibody molecules and pours them into the bloodstream.
Source: National Cancer Institute, USA.
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