ObjectiveTo investigate the causal relationship between 731 kinds of immune cell phenotypes and positive-human epidermal growth factor receptor (HER+), negative-human epidermal growth factor receptor (HER–), negative-human epidermal growth factor receptor 2 (HER2–) breast cancer. MethodsGenome-wide association data using immune cell phenotype and breast cancer were used with inverse variance weighting as the primary analytical method; horizontal pleiotropy was tested using MR-PRESSO and outliers were corrected; in addition, the reliability of the obtained data was verified using Cochran’s Q test, Mendelian randomization (MR)- Egger regression and leave-one-out method were used to verify the reliability of the obtained data. ResultsFor HER+ breast cancer, CD3 on CD39+CD4+T cell [ OR=0.940, 95%CI (0.913, 0.968), P=0.019] was protective factor. For HER– breast cancer, no immune cell phenotype was found to be correlated with it. For HER2– breast cancer, CD3 on CD39+CD4+ T cell [OR=0.951, 95%CI (0.930, 0.973), P=0.014], CD3 on secreting CD4 regulatory T cell [OR=0.949, 95%CI (0.925, 0.974), P=0.023] were protective factors. ConclusionThere is a causal association between certain immune cell phenotypes and breast cancer, which may be predictive markers for early diagnosis of breast cancer and development of new immunotherapies.