Natural Killer Cell-Mediated Resistance to Animal Parasites

Abstract

The expression “natural resistance,” or even “natural cell-mediated resistance,” can be applied to a variety of mechanisms including those that involve macrophages, polymorphonuclear cells, and, in a broad sense, platelets, and that may involve humoral substances such as components of the complement system.(1) By inserting the word “killer” in the title of this article we hope to convey an understanding that this article will deal with a reasonably proscribed subject—viz., the role of natural killer cells in resistance to parasites. The term “natural killer” will be considered imprecise by some workers who will be correct in raising an objection to its use. We might borrow the expression “non-MHC-restricted cytotoxicity,” (20) but that also suffers from lack of precision. Because at the present time, there is no satisfactory term, we will continue with the term “natural killer” (NK) and stress the fact that, in much of the work that will be reviewed, it was an activity rather than a well-defined cell that was under investigation. The expression “NK cells” will not be used as a synonym for large granular lymphocyte (LGL). Even in those investigations to be discussed where concerted effort was made to associate phenotypic markers with NK activity, the precise identity of the active cell(s) must remain in doubt. That is because, by any set of criteria, NK cells are heterogeneous,(3-5) and there are insufficient data to allow a decision about the number of different subsets of NK cells, let alone their classification.