Objective To explore the causal relationship among 731 types of immune cells and breast cancer.Methods Genome-wide association data for immune cells and breast cancer were used.Mendelian randomization analysis was performed using the inverse variance weighting(IVW)and weighted median(WM)methods,and sensitivity analysis was conducted to assess heterogeneity and pleiotropy.Results A total of 19 immune cell phenotypes were identified to potentially have a causal association with breast cancer,using IVW as the main analysis method(P>0.05)and correcting P values using the false discovery rate method at a significance level of 0.05,excluding reverse causality.Of these,eight and 11 immune phenotypes may increase and decrease the risk of breast cancer,respectively.Conclusion This study explored the causal relationship between immune cells and breast cancer.Results show that certain immune cell phenotypes could serve as predictive markers for the early diagnosis of breast cancer and the development of new immun-otherapeutic strategies.