[Immunity in the fetus and the newborn infant: a swine model].

An efficient placental barrier protects the pig fetus from all exogenous antigenic stimulation. The results of studies on different subpopulations of lymphocytes, histological localization, and stimulation as well as immunization in utero with allogeneic lymphocytes have confirmed that the pig fetus becomes immunocompetent at around 80 days of fetal life (total length of gestation: 115 days). Because the newborn piglet has had no "immunological experience" in utero, it can only elaborate a primary response which is often advantageous to pathogenic microorganisms. Under natural conditions, immune protection is provided by the mother in the form of colostrum. This colostrum provides systemic immunity and is followed by milk which insures local immunity in the piglet gastrointestinal tract. However, the great disadvantage of maternal antibodies is that they suppress the active synthesis of new antibodies in young pig. Thus, the beginning of active immunity in the piglet is a function of the ratio of maternal antibody level to the level of antigenic stimulation.