In origin immunoglobulins in mammary secretions are both humoral, arising from the blood stream, and local, arising from production by plasmacytes in the mammary gland. The relative importance of each of these sources varies between species. In some species (human, rabbit, etc.), the transfer of maternal immunoglobulins to the blood stream of the neonate occurs in utero across the placenta or yolk sac membrane. In other species, including ruminants, transfer of maternal immunoglobulins to the neonate occurs exclusively via the colostrum. Both in utero and colostral routes of transfer are operative in other species. The concentration and class of immunoglobulins in the colostrum and milk of a species reflect the route and origin of the immunoglobulins. Immunoglobulins transferred in quantity in utero or via the colostrum are mainly of the IgG class. Immunoglobulins locally produced by plasmacytes located adjacent to the secretory epithelium and in the mammary secretions are largely of the IgA and IgM classes. The bovine transfers large amounts of IgG immunoglobulins, and IgG1 in particular, from the blood stream across the mammary barrier into colostrum (and milk) by a specific transport mechanism. Bovine colostrum and milk also contain much smaller amounts of locally produced IgA and IgM.

Production rates (entry rate into blood plasma) and other metabolic parameters for the IgG1 and IgG2 subclasses of immunoglobulins in mammary secretions (ratio of about 7 to 1) were determined in cows around the time of parturition by both single-injection and continuous-infusion isotope-dilution techniques. Four cows were given a single dose of 150 to 200 muCi of iodine-125 labeled IgG1 and 100 to 250 muCi of iodine-131 labeled IgG2 at 2 to 1 wk before parturition. Four cows, including two of the above, were infused continuously with constant amounts of the labeled immunoglobulins starting at 11 to 4 days before parturition. All cows were maintained until 4 to 6 days after parturition for monitoring the specific activities of iodine-125 labeled IgG1 and iodine-131 labeled IgG2 in the plasma and mammary secretions. Maximum entry rates of IgG1 and IgG2 were between 3 and 1 day prepartum with means of 125 and 60 g/500 kg body weight per day. The exchangeable pool means for IgG1 and IgG2 were 619 and 643 g/500 kg body weight, and both immunoglobulins were divided almost equally between the intra- and extravascular pools. A greatly increased production and a shortened half-life or greater turnover for plasma IgG1 occurs around the time of parturition which can account for the large accumulation of IgG1 in colostrum.

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Blood and colostrum samples were obtained from 87 dairy cows in five lactation groups and analyzed for immunoglobulins G1, G2, M, and A. The five groups ranged from cows in first lactation, about 30 mo of age, to cows in fifth or more lactation, about 84 mo of age. Compared to older groups, blood serum of cows in first lactation contained less G1. Cows in first lactation also produced less total colostrum containing less total G1, G2, and M. Immunoglobulin G1 comprised over two-thirds of the immunoglobulins in the colostrum of all groups. Older cows had more immunoglobulin G1 in their colostrum with a tendency toward a higher ratio of G1 to G2. Amount of immunoglobulin A was constant through all lactations. After a rise in the second lactation, total amount of immunoglobulins G2 and M tended to level off. Total immunoglobulin G1 tended to reach a maximum in the third or fourth lactation, almost doubling in amount compared to the first lactation. Age and number of lactations are factors correlated with amounts of these immunoglobulins in colostrum.

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