Why is the fetus not rejected as foreign tissue? The maternal and fetal immune systems temporarily coexist; both are precisely tuned to detect and reject foreign invasion and yet somehow achieve a symbiotic relationship. This mutual state of tolerance is obviously critical for carrying pregnancy to full term. Two active arms of the immune system maintain protection of the host: the first of these involves a humoral immune system in which foreign tissue invokes an antibody response by recognition of antigenic surfaces by the B-cell, the second arm involves cell-mediated immunity in which T-cells and natural killer (NK) cells seek out and destroy foreign tissue. Several mechanisms are thought to invoke the immune tolerance of the fetus. These include: absence of major histocompatibility complex (MHC)-I antigens, presence of unique human lymphocyte antigen (HLA) surface molecules, nonspecific reduction of systemic immunoreactivity, possible role of blocking antibody, expressions of complement regulatory proteins, and factors of locally reduced immunoreactivity. Ultimately, developing regimens to control these elements in the clinical setting may help us overcome preterm labor, infertility, and preeclampsia. Available evidence regarding immune tolerance of the human fetus, integrated into a workable model, and focused at an overview level are systematically reviewed in this article.