Experimental work in immunogenesis utilizing intrauterine surgery supports the hypothesis that the development of immunologic competence may begin during the early stages of pregnancy in the mammalian fetus.
It is generally believed that immunologic competence may not be apparent only because the fetus is not exposed to antigenic stimulus under normal conditions in the uterus.
Research at the Armed Forces Institute of Pathology, Washington, DC, recently demonstrated that as early as the 41st day of a 150-day gestation, fetal lambs produce circulating antibodies in specific response to antigenic stimulus by bacteriophage φX. Substantial amounts of antibody to horse ferritin were elicited at 65 to 70 days. Other agents were less effective, or completely ineffective during gestation or the early neonatal period.
Another type of immunologic response, skin homograft rejection, thought to be a cellular response related to delayed hypersensitivity, was demonstrable in fetal lambs at approximately midterm, but not before.