Ontogeny of host defense systems and congenital infections.

There are a number of factors which contribute to the large number of congenital infections which occur in domestic animals. The more important factors include: 1) The agammaglobulinemic fetus because of a lack of transplacental transfer of maternal immunoglobulins, 2) An immature immune and accessory immune system, and 3) Undifferentiated cells in a number of developing organ systems in the fetus. The immune and accessory immune systems and other organ systems develop in a sequential manner. The types of lesions associated with a microbial agent depends upon the status of the target organ, immune response and accessory immune response at the particular moment of the insult. Once immune competence is attained, the fetus clears infection. Immune tolerance occurs with certain pestiviruses which have a predilection for lymphoid cells. Infection must occur before immune competence to these viruses is attained.