[Nonspecific natural cytotoxicity as an active participant in the ensemble of the means of immune defense].

The organism of mammals, the most evolved animals phylogenetically possess complex mechanisms of immune defence. The effectors of these mechanisms can be grouped according to the way in which they recognize structures non self to the organism, into two large categories: (a) that attack the antigen regardless of its antigenic nature, therefore unspecific, and (b) that attacks and destroys the antigen selectively, i.e. specifically. According to the way in which and the distance at which foreign elements are recognized, the defence means act (a) by humoral mediated (at a distance through antibodies), and (b) by cellular mediation (by direct contact between the effector cells and the target). Until recently, the humoral mediated defence (formation of antigen-antibody complex, complement activation, etc.) or some forms of cell mediated defence (cytotoxicity exercised by T cytotoxic cells or K killer cells) were considered as the major, dominant elements of protection of the organism. Another defence component has recently been identified, that acts unspecifically by direct contact with the target cell and which, in contrast to what was known up to date, manifests its function even if the organism has never before come in contact with the respective aggressor (was not immune stimulated). This means of defence is not influenced by previous immunizations, being maintained at a constant level, but is influenced by a series of other factors. It is known as unspecific natural cytotoxicity, and the effector cells as natural killer (NK) cells. The NK function intervenes along the first line of defence in viral, bacterial infections, neoplasms, etc. The present paper refers to the biological role of NK cells and their possible relationship with the other components of the immune system.