13440
This observation prompted us to postulate that signal 2 ( costimulatory signal ) alone without signal 1 ( TCR signal ) can activate T cells .
 TCR, T 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:
 TCR, T cells