develops from natural killer cell immature natural killer cell An immature form of a natural killer cell, a lymphocyte that can spontaneously kill a variety of target cells without prior antigenic activation via germline encoded activation receptors and also regulate immune responses via cytokine release and direct contact with other cells. Immature natural killer cells have begun to express the characteristic natural killer cell activation and inhibitory molecules through a selection process leading to tolerance to normal self cells. CL:0000823 TAO:0009346 p-NK teleost_anatomy natural killer cell progenitor