Lymphocytes are a type of no granular white blood cells in the vertebrate immune system. The three major types of lymphocytes are T cells, B cells and natural killer (NK) cells: once T cells (thymus cells) and B cells (bone cells) have identified an invader, B cells produce antibodies which neutralize foreign objects and T cells generate cytokines that direct the immune response and toxic granules that kill the pathogen infected. Once they have actuated once, they leave antigens that detect any foreign object that have attack the body before. NK cells distinguish infected cells and tumors from normal cells and destroy them.