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Moreover , our previous studies showed that the immunosuppressive effects of ptaquiloside , its main toxic agent , were prevented by selenium in mouse natural killer ( NK ) cells .
 Chemical: ptaquiloside; Chemical: selenium
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: ptaquiloside; Chemical: selenium