Women with proportions of natural killer cells (HR, 1.03; 95% CI, 0.89-1.20; P = .70), CD8+ T cells (HR, 1.07; 95% CI, 0.92-1.24; P = .38), and CD4+ T cells (HR, 1.09; 95% CI, 0.94-1.27; P = .26) greater than the subcohort median had higher breast cancer risks, but the associations were not statistically significant. Here, CD4 is linked to breast cancer.