4151
A major obstacle in understanding the signaling events that follow CD28 receptor ligation arises from the fact that CD28 acts as a costimulus to TCR engagement , making it difficult to assess the relative contribution of CD28 signals as distinct from those of the TCR .
 protein: TCR; protein: CD28; protein: TCR; protein: CD28; protein: CD28
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: TCR, CD28, TCR, CD28, CD28 
Answer:
 protein: TCR; protein: CD28; protein: TCR; protein: CD28; protein: CD28