Among the 15 immune cell phenotypes with causal relationships identified through MR analysis, only a subset (CD4/CD8br on T cells, CM CD4+ AC, CD19 on IgD− CD24−, CD19 on IgD− CD27−, and CD38 on IgD+ CD24− on B cells) are likely to contribute to BC development through blood-derived metabolic mediators. The gene discussed is CD4; the disease is breast cancer.