Natural killer cells do not belong to the recirculating lymphocyte population.

Natural Killer (NK) cells constitute a cell type with an as yet undefined lineage, although certain similarities with T lymphocytes have been found in the mouse. Our present results show that NK cells have a significant difference compared with T and B cells in their capacity to traverse the blood-lymph barrier. Thoracic duct lymphocytes from mice or rats are thus devoid of NK activity, when at the same time potential, cytotoxic T lymphocyte (CTL) activity or antibody-dependent cellular cytotoxicity function can be demonstrated. Chronic thoracic duct drainage in the rat also leads to an increase in NK activity per unit cell number in the other lymphoid organs. Thoracic duct lymphocytes from mice and rats may thus serve as convenient sources of CTL and/or killer cells in situations in which it is important to minimize NK cell involvement.