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The contact of natural killer (NK) cells with foreign cells and with certain virus-infected or tumor cells triggers the cytolytic machinery of NK cells .
 foreign cells, NK cells, natural killer (NK) cells
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: cell line, protein, RNA, DNA, cell type 
Text: {0} 
Answer:
 foreign cells, NK cells, natural killer (NK) cells