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Intestinal Absorption of Immunoglobulins


Intestinal
Absorption

Piglets suckling a sow.

Normally we think of ingested food as being degraded in the digestive tract and digestion products being absorbed into the animal. For example, proteins would be degraded to amino acids or small peptides and those are absorbed. However, for a short time after birth most mammals will absorb macromolecules intact. This is particularly important for absorption of colostral immunoglobulins which would be rendered inactive if digested. This absorption occurs by a nonspecific pathway (not receptor mediated) where macromolecules are taken into the intestinal absorptive cells (enterocytes) by formation of tubules at the base of the apical microvilli. These tubules pinch off in the cell to form small vesicles that can transport the contents to the basolateral membrane and release their contents into the extracellular space. From there the macromolecules can be absorbed into the blood. This process of macromolecule absorption only occurs in the jejunum, not in the ileum. Macromolecules are taken up into ileal enterocytes, but are degraded in lysosomes within the cell. In those cases where intestinal absorption is not selective, most anything will be absorbed including carbon aggregates and plastic. Therefore, there is little discrimination in absorption of the different immunoglobulins.

Intestinal transport of macromolecules is extensive but not selective in some species (such as the bovine, porcine and canine neonate), while in others (human infants, guinea pigs, rabbits) there is little intestinal absorption of macromolecules at all. In the latter cases, maternal immunoglobulins are transported to the fetus before birth and the neonate is born with high concentrations of immunoglobulins inthe blood. Rats, and mice are an intermediate group because, in addition to the limited intestinal nonselective uptake after birth, there is a highly specific receptor-mediated intestinal absorption of IgG that continues for about the first 20 days of life.

The diagram below illustrates the temporal relationship between closure, IgG absorption, colostral IgG, and calf serum IgG. The process of macromolecular absorption is initially high at the first suckling, then declines gradually. Intestinal closure to uptake of macromolecules has occurred when no more intact macromolecules can be absorbed. Intestinal closure is a continual, gradual process that starts immediately after birth and proceeds until there is no longer transport of macromolecules. Time of closure is the time after birth when macromolecules (including immunoglobulins) can no longer pass from the intestinal lumen, through the intestinal cell and into the neonate's vascular system. Closure is complete in the calf by about 24 hr after birth, in the piglet at about 36 to 48 hr, in the foal at about 24 to 48 hr, in cats and dogs at about 24 to 48 h.

Diagram illustrating relationships between closure, IgG absorption, and colostral IgG.

 

See Staley and Bush, 1985, J. Dairy Sci. 68:184; Bush and Staley, 1980, J. Dairy Sci. 63:672; Larson et al., 1980 J. Dairy Sci. 63:665.


 
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