Ontogeny of the immune system.

Cellular and humoral aspects of the immune response develop sequentially in the fetus. Although there is evidence of immunologic activity to some antigens, many of the nonimmune effector systems are neither developed nor fully functional in the fetus. The lack of fully functional systems make the fetus a highly susceptible candidate for invading bacteria and viruses. Often, infection prior to immune competence may lead to abortion, malformation, and in some instances, viral persistence and immune tolerance. On the other hand, immune responses tend to elevate immunoglobulin values and cause specific antibody that may be used for diagnostic purposes. Little work has been done to appreciate fully the immunologic and nonimmune effector systems' role in normal development or during congenital infections.