The NK cells have a cytotoxic activity against cells infected by viruses and tumor cells, although they do not recognise them by a specific antigenic stimulus (the NK cells do not express TcR). Neither are they limited by the SLA, as occurs with T lymphocytes (CD 8). Their activation mechanisms begin when they naturally recognise cells that do not adequately express SLA, as is the case in most cells infected by a virus. In addition, they release soluble mediators (cytokines), such as Tumour Necrosis Factor (TNF), interferon g. This allows the stimulation of various lymphoid mechanisms.

In porcine species, the NK cells are small or medium-sized lymphocytes without granules, in contrast to murine and human species in which the NK cells are large, granular lymphocytes. Porcine NK cells react with monoclonal antibodies against porcine T lymphocytes belonging to the CD 2 and CD 8 groups , but not with those of CD 3, CD 5 and CD 6 groups. Furthermore, the reaction with CD 8 is much lower than that of the cytotoxic T lymphocytes CD 8+. The best monoclonal antibody for the study of porcine NK cells corresponds to the CD 16 group.