A third class of lymphocytes are called natural killer (NK) cells. NK cells are not dependent on the thymus, and they can attack certain types of specific cells without prior sensitization.
Natural Killer cells constitute 10-15% of lymphocytes in the peripheral blood. They are neither T nor B Cells and were formerly called null cells. They respond to tumor cells and intracellularly infected cells and are capable of antibody-dependent cell-mediated cytotoxicity (ADCC), and thus can lyse infected cells. NK cells often appear as large granular lymphocytes, although this distinction is not made when reporting WBC differentials.