ERBB2 and breast cancer: This study represents the first investigation employing 2-sample Mendelian randomization (MR), multi-marker analysis of genomic annotation (MAGMA), Metascape, and the Kaplan–Meier (K–M) plotter database to elucidate the causal relationship between immune cells (ICs) and human epidermal growth factor receptor 2 negative breast cancer (HER2-BC).