IgG is the most abundant immunoglobulin in colostrum, followed by IgA and IgM. Colostral immunoglobulins are exposed to enzymatic processes in the oral cavity and gastrointestinal tract of the newborn. IgGs are the most subject to degradation, IgAs and IgMs being less labile because of their association with a secretory component (SC). Some protection against enzymatic breakdown of immunoglobulins may be provided by a trypsin-inhibitory factor present in colostrum, coupled with the fact that the enzymatic activity of pancreatic and intestinal secretions in the neonate are much lower than they will become later in life.
The gut of certain neonates can absorb large amounts of intact protein as a means of antibody transfer i.e. acquisition of passive immunity (whereas the healthy adult intestine can absorb only small and nutritionally insignificant amounts of intact protein). The mechanism of protein absorption in the neonatal gut is endocytotic i.e. it occurs within the cell. The first step involves adsorption of macromolecules to receptors for the Fc portion of the immunoglobulin in the microvillous surface of the small intestinal cell. When the macromolecules have reached a critical concentration at the microvillous border, the membrane invaginates and small vesicles are formed. This process requires energy for the replacement of the cell membrane. Now the membrane-bound vesicles (phagosomes) migrate to the supranuclear region of the cell, and here they coalesce with lysosomes to form large vacuoles called phagolysosomes these vesicles, intracellular digestion occurs, but some of the macromolecules survive intact and migrate to the basolateral surface of the cell membrane. Here exocytosis occurs, and the macromolecules pass into the intercellular space and thence into the lymphatic system.
In ruminants, pigs and horses, all the maternal immunoglobulins received from the colostrum are transferred across the intestine by the process described above. These animals have very low levels of gamma -globulins at birth, and for a short period after birth the intestinal mucosa is permeable to all immunoglobulin uptake by the intestine lasts for only a short time; the intestine is then said to undergo "closure" to prevent further transfer of proteins. There appear to be factors present in the colostrum which initiate the process of closure.
Immunoglobulins acquired neonatally protect the newborn in two ways: (1) they provide circulating antibody to help prevent systemic disease, and (2) they provide antibody at the level of the intestinal epithelial cell to block the subsequent attachment of the potential pathogenic organism to target cell.
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