PROBLEM: This study investigated how pregnancy might protect against b
reast cancer. METHOD OF STUDY: A critical review of the literature was
done. RESULTS/CONCLUSIONS: Support for an active role in pregnancy im
munizing against breast cancer comes from case studies demonstrating a
reciprocal correlation between pregnancy and breast cancer as well as
recent experiments supporting the fetal antigen hypothesis that confi
rms the presence of a tumor-specific antigen, MUC1, on both fetal and
breast cancer tissues. Multiparous women also generate anti-MUC1 major
histocompatibility complex-restricted cytotoxic T cell cytolytic acti
vity against MUC1-bearing tumor cell lines. Careful investigation of t
he fetal antigen hypothesis and tolerogenic mechanisms may lead to eff
ective vaccination protocols against breast cancer and other cancers.