The epitheliochorial placenta of the pig does not allow the passage of immunoglobulin to the foetus and thus the young piglet is born without passive immune protection. During the first 36 h of life there is a massive transmission of macromolecules across the intestine, virtually all that are present in the gut may be effectively endocytosed and transmitted into the blood stream. The postnatal transmission of antibody from colostrum during this period provides the young animal with a spectrum of serum antibodies indistinguishable from that of its mother. It is established that even in utero the piglet is capable of mounting some response to antigenic challenge. Despite this, the ability of the young animal to respond may be influenced profoundly by the absorption of macromolecules (antibodies and antigens in colostrum and in sow's milk as well as antigens in the farrowing house) during the first hours after birth. These effects range from passive protection from infectious agents during the neonatal period to determining the precise nature of the immune response to antigens during later periods (e.g. at weaning).
At birth all cellular components of the immune system are represented but during the first few weeks of life dramatic changes occur in the number and distribution of these cells. Our histological studies have shown that shortly after birth the predominant T lymphocytes in the small intestine are T2+, T4− and T8−; whilst in other organs there are large numbers of conventional T4+ and T8+ cells. By 1 week of age there is a dramatic increase in the numbers of T4+ cells, whilst T8+ cells remain low, and only start to increase by week 7. Thus, changes in lymphocyte populations are occurring concurrent with increasing exposure to environmental antigens. The functional capacity of these cells also changes during this period and this process may be particularly affected by early weaning.
During the neonatal period an animal is presented with a vast array of antigenic material for the first time. How and when these antigens are presented may profoundly influence the capacity of the immune system to respond to them.