15470
In humans , there is a single receptor for C3a ( C3a receptor ) , no known receptor for C4a , and two receptors for C5a ( C5a 1 receptor and C5a 2 receptor ) .
 None
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 None