Abstract: In order to investigate the effects of p-nitrophenylalanine on the differentiation of T lymphocyte, human epidermal growth factor receptor 2(HER2)mutants were constructed by incorporating p-nitrophenylalanine to the extracellular domain of HER2. Mice were immunized with HER2 mutant to analyze the effects of p-nitrophenylalanine on the differentiation of T cell subsets. The results showed that the immunogenicity of HER2 protein was significantly enhanced by incorporation of immunogenic amino acids. HER2 mutants induced production of more anti-WT-HER2 antibody and greater titer of antibody than PBS and wild-type HER2 in the fifth week. The differentiation frequency of Th1 and Th2 cells showed no significant difference, but the differentiation frequency of Tfh cell was found to increase from 4% to 8%, while the differentiation frequency of Treg cell was found to decrease from 5% to 1% through flowcytometry analysis. The gene expression of Bcl-6 and FOXP3 were determined by quantitative real-time PCR. The results showed that the expression of Bcl-6 mRNA increased about 2 fold, while the expression of FOXP3 mRNA could be decreased to about 50% in HER2 mutant-immunized group. The secretion level of IL-21 in serum was also detected to increase from 4 to 28 pg/mL in HER2 mutant-immunized group. The above results confirm that HER2 mutants containing p-nitrophenylalanine can cause an efficient immune response by activating Tfh cell differentiation.