Human NK cells are classified into two major subsets based on their CD56 surface expression. Most circulating blood NK cells are CD56dim, while 5–15% are CD56bright. CD56bright NK cells are poorly cytotoxic but secrete high levels of cytokines and chemokines in response to inflammatory cytokines. Although CD56dim NK cells respond weakly to inflammatory cytokines, they kill target cells (such as the erythroleukemia cell line K562) and secrete chemokines and cytokines in response to antibody-coated cells and tumor cells.