17681
The direct killing of target cells by cytotoxic T lymphocytes ( CTLs ) plays a fundamental role in protective immunity to viral , bacterial , protozoan and fungi infections , as well as to tumor cells .
 None
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 None