By 1973, 'natural killing' activity was established across a wide variety of species, and the existence of a separate lineage of cells possessing this ability was postulated.
Porén, en 1973, a actividade "asasina natural" quedou establecida despois de múltiples experimentos en varias especies de seres vivos, e postulouse a existencia dunha liña especial de células que posuían esta capacidae.