The influence of pregnancy on breast cancer risk: is it endocrinological or immunological?

Epidemiological data are presented to show that pregnancy is related to a short-term increase in breast cancer risk which precedes its well recognized lower rate for the remainder of life. The data confirm and extend related findings from several earlier studies, and suggest that the biological mechanism which produces the apparent protective effect resulting from pregnancy also produces a short-term adverse effect which is approximately proportionate to the protective effect. This phenomena seems important in terms of what it reveals about the natural history of breast cancer. It is proposed that the general biological mechanism underlying this effect is the immune tolerance during pregnancy which allows the female's immune system to tolerate a fetus whose genes and antigens are partially dissimilar to her own. It is further suggested that the specific mechanism responsible for the phenomena is either a mammary tumor virus or the woman's immune reaction to unidentified fetal antigens that are common to human fetal cells and mammary cancer cells.