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The reason that only certain individuals of particular major histocompatibility complex (MHC) haplotypes can mount T-cell responses against a given antigen of pathogens is found in the fact that T-cell receptors are designed to recognize 18-20 residue-long peptide fragments sandwiched between two alpha-helices of class I or class II MHC molecules .
 protein: class II MHC molecules; protein: alpha-helices; protein: T-cell receptors; protein: major histocompatibility complex (MHC) haplotypes; protein: class I
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: cell line, protein, RNA, DNA, cell type 
Text: {0} 
Answer:
 protein: class II MHC molecules; protein: alpha-helices; protein: T-cell receptors; protein: major histocompatibility complex (MHC) haplotypes; protein: class I