Lymphocytes are a type of white blood cell primarily involved in fighting foreign pathological agents that mature and develop in lymph organs such as the bone marrow and thymus gland. A large number of lymphocytes can also be found in the spleen and lymph nodes, as well as other lymphatic tissues at essential stations throughout the human body. There are three principle types of activated lymphocytes, which are B-lymphocytes, T-lymphocytes, and natural killer cells (NK). B-lymphocytes and T-lymphocytes are commonly referred to as B cells and T cells, respectively. Taken together these cells form the arsenal of the immune system.
Antibody-mediated immunity is directly related to B-cells, in which a B-cell produces copies of one type of antibody, a molecule acting as a surface receptor on a cell that binds with an antigen specific to itself. Once the binding has occurred, the cell is activated and the antigen is broken down into peptide parts, which is then displayed on the cell's surface. Activated lymphocytes of the B cell type enlarge, undergoing mitotic cell division yielding exact clones. Each b cell clone produces antibodies specific to the particular antigen that activated the b-lymphocyte of origin.
T cells are activated lymphocytes necessary to cell-mediated immunity in which T-lymphocytes act to destroy cells that they recognize as infectious or abnormal in some way, as with cancerous cells. Every cloned T cell has many receptors that are identical to itself that bind to only one specific antigen. These cells are activated to attack upon recognition once the degraded peptide fragments are represented in a particular way on the cell's surface. Helper T cells are produced, which then leave the lymph nodes to the area of infection for the purpose of attacking infectious cells. Serving as signals for immune responses, cytokines are produced by helper t cells in the form of interleukins, tumor necrosis factors, and interferons, which are released by macrophages.
Natural killer cells are also activated lymphocytes important to the destruction of a variety of infectious agents, such as bacteria, fungi, and viruses. Activated primarily by the release of cytokines, NK cells are derived from dividing T-lymphocytes and are sometimes called "killer T cells." When killer cells engage with an antigen on the surface of an infected cell, granules containing cytotoxic proteins are released, which attack and kill the cell. After the infected cell succumbs, the NK cell moves on to identify another infectious agent.