Blood type A and familial breast cancer.

The association between blood type A and breast cancer was evaluated in 648 patients with family histories of the disease, 1897 unselected patients, 4577 institutional blood donor controls, and 14,508 extramural blood donor controls. The familial patients were classified into three pedigree groups in which the lifetime breast cancer risks to first-degree relatives ranged from 11% to 32%. In the pedigree group associated with a relatively low risk to relatives, the authors saw a significant excess of blood type A individuals when compared with either control group. The unselected patient population, which was considered to refer to a general series of patients, also showed an excess of blood type A individuals with either control but a significant excess only in comparison with institutional controls. Patients from high-risk pedigrees with inherited forms of breast cancer showed no association when compared with either control group. The effect of blood type A on breast cancer risk was considered too small to be of use in identifying women at high risk. The results further suggested that the effect of blood type A on breast cancer development was capable of being masked by the effect of breast cancer susceptibility genes and/or that the inherited and noninherited types involve different etiologic mechanisms.