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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 .
 cell type: foreign cells; cell type: NK cells; cell type: natural killer (NK) cells
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: cell line, protein, RNA, DNA, cell type 
Text: {0} 
 Entities: foreign cells, NK cells, natural killer (NK) cells 
Answer:
 cell type: foreign cells; cell type: NK cells; cell type: natural killer (NK) cells