Sustained lymphocyte decreases after treatment for early breast cancer.

The role of adaptive immunity in long-term outcomes in early breast cancer is increasingly recognised. Standard (neo)adjuvant chemotherapy can have adverse effects on immune cells. We conducted a retrospective longitudinal study of full blood counts (FBC) of 200 patients receiving (neo)adjuvant chemotherapy for early breast cancer at a single institution. FBC results at four time points from pre-treatment to 12 months post-chemotherapy were analysed. Flow cytometry was performed for patients with matched pre- and post-chemotherapy peripheral blood mononuclear cell samples. A significant decrease in absolute lymphocyte count at 12 months post-chemotherapy was observed (p < 0.01), most pronounced in pre-menopausal patients (n = 73; p < 0.01), patients receiving dose-dense chemotherapy regimens (n = 60; p < 0.01) and patients receiving adjuvant radiotherapy (n = 147, p < 0.01). In pre-menopausal patients, significant changes in CD4⁺ T cells subsets post-chemotherapy were observed. Further investigation, including long-term clinical outcomes, is needed to meaningfully improve long-term anti-tumour immunity.