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Immunoglobulin Transport in the Mammary Gland


Piglets suckling a sow.

The young of most and perhaps all mammalian species do not develop an effective immune system until after birth. Humeral immune protection (immunoglobulins) is supplied to the neonate by a process of transfer of passive immunity from the mother to the neonate. This generally occurs by transfer of maternal serum IgG from the mother to the offspring either in utero or, after birth, by ingestion of immunoglobulin-rich colostrum by the neonate. These maternal immunoglobulins offer immune protection until development of a competent immune system in the neonate and even may be involved in modulating the neonate's developing immune system. In species where transfer of immunity occurs via colostrum, the lack of colostrum intake shortly after birth can lead to neonate mortality rates approaching 100%. This process of transfer of immunoglobulins from mother to the young is of paramount importance to neonate survival.

Immunoglobulin isotype G1 (IgG1) is the major immunoglobulin transported by the cow mammary gland during colostrum formation. Specific transport of IgG2 also may be increased some. The IgG1 and IgG2 make up the majority of immunoglobulin in cow colostrum and primarily come from the blood (that is they are pre-formed). Most of the IgA and IgM that are transported into colostrum are synthesized by the plasma cells (B lymphocytes) that reside in the mammary tissue. Transport of the IgGs and the IgA/IgM occurs through the epithelial cells by a process involving small transport vesicles. However, the receptors for the IgGs and the IgA/IgM are different receptors. The receptor for IgA/IgM is called secretory component (SC) and is proteolytically cleaved off the membrane during transport of the IgA. The SC remains bound to the IgA and the SC-IgA complex is called secretory IgA. There is also a lot of non-bound SC in milk and colostrum, suggesting that the proteolytic cleavage of SC does not require that it be bound to IgA. The receptor(s) for IgG tr aport has not been completely identified at this time.

Transport of maternal immunoglobulins into colostrum probably occurs in all mammals to varying extents, but the significance of the immunoglobulins in colostrum depends on the species. Humans and other primates transport immunoglobulins to the fetus through the placenta via a receptor-mediated, intra-epithelial mechanism similar to that in the mammary gland. Therefore, when the infant is born it already has a full complement of immunoglobulins in its blood to protect it for disease until its own immune system is fully functional. Transport of immunoglobulins into colostrum in primates does occur (primarily IgA/IgM) but to a more limited extent. However, in most species immunoglobulins are not transported across the plactenta, therefore the colostral immunoglobulins are critically important to neonate survival. This is the case in the domestic farm species. In the dairy cow, as much as 2 kilograms of IgG can be secreted into the colostrum during the first five milkings. Another exception is the rat which transports some immunoglobulin across the placental yolk sac and some via the colostrum.

After ingestion of colostrum by the neonate, the immunoglobulins are absorbed intact into the neonate's blood stream. This process of immunoglobulin absorption in the intestine stops after a time postpartum depending on the species. This halt in intestinal absorption of immunoglobulins and other macromolecules is called closure.

Immunoglobulin concentrations decline rapidly over the first 24 hr after parturition. The total amount of immunoglobulins secreted in the colostrum increases with parity of the mother. For example, the first lactation cows will have about one half the IgG1 concentration that third and fourth lactation cows will have. Concentrations of colostral IgG2 and IgM also are lower in first lactation cows, while the concentration of IgA is only slightly lower.

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