In this study, label-free quantification analysis and gene set enrichment analysis (GSEA) of CD8<sup>+</sup> T lymphocytes in the peripheral blood of benign patients and patients with different breast cancer (BC) subtypes, i.e., luminal A, luminal B, and triple-negative breast cancer (TNBC), were performed using nano-UHPLC and Orbitrap mass spectrometry. Here, CD8A is linked to breast cancer.