Classical thinking suggests that the immune system undergoes activation on the basis of discrimination between 'self' and 'non-self'. Accordingly, the fetus activates the mother's immune system because the fetus is in part 'non-self'. Thus, successful pregnancy depends on constraint of maternal immunity. Preeclampsia is an outcome of lost constraint. Instead, the danger model suggests that normal pregnancy, regardless of the expression of 'non-self' antigens, does not activate the maternal immune system unless that pregnancy expresses danger signals. Thus, preeclampsia stems from stress or abnormal cell death in pregnancy-related tissues. This compels expression of specific danger signals and potential activation of anti-fetal immunity, which secondarily feeds the syndrome. Study of preeclampsia from this perspective may bring forth novel mechanisms and indicators of vascular and metabolic dysfunction during pregnancy.