Colostrum and milk provide a complete diet for the neonate. In ruminants, colostrum is also the sole source of initial acquired immunity for the offspring. Milk therefore plays an important role in mammalian host defense. In colostrum, the concentration of immunoglobulins is particularly high, with IgG being the major immunoglobulin class present in ruminant milk, in contrast to IgA being the major immunoglobulin present in human milk. Immunoglobulins are transported into mammary secretions via specialized receptors. In addition to immunoglobulins, both colostrum and milk contain viable cells, including neutrophils and macrophages, which secrete a range of immune-related components into milk. These include cytokines and antimicrobial proteins and peptides, such as lactoferrin, defensins, and cathelicidins. Mammary epithelial cells themselves also contribute to the host defense by secreting a range of innate immune effector molecules. A detailed understanding of these proteins and peptides offers great potential to add value to the dairy industry. This is demonstrated by the wide-ranging commercial applications of lactoferrin derived from bovine milk. Knowledge of the immune function of milk, in particular, how the gland responds to pathogens, can be used to boost the concentrations of immune factors in milk through farm management practices and vaccination protocols. The latter approach is currently being used to maximize yields of bovine milk-derived IgA directed at specific antigens for therapeutic and prophylactic use. Increasingly sophisticated proteomics technologies are being applied to identify and characterize the functions of the minor components of milk. An overview is presented of the immune factors in colostrum and milk as well as the results of research aimed at realizing this untapped value in milk.

Besides providing nutrition to the newborn, milk also protects the neonate and the mammary gland against infection. As well as the six major proteins, bovine milk contains minor proteins, not all of which have been characterized. In this study, we have subjected bovine skim milk, whey, and milk fat globule membrane (MFGM) fractions to both direct liquid chromatography-tandem mass spectrometry (LC-MS/MS), and two-dimensional electrophoresis (2-DE) followed by matrix assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometry (MS) of individual protein spots to better characterize the repertoire of minor milk proteins, particularly those involved with host defense. Milk from peak lactation as well as during the period of colostrum formation and during mastitis were analyzed to gain a more complete sampling of the milk proteome. In total, 2903 peptides were detected by LC-MS and 2770 protein spots by 2-DE. From these, 95 distinct gene products were identified, comprising 53 identified through direct LC-MS/MS and 57 through 2-DE-MS. The latter were derived from a total of 363 spots analyzed with 181 being successfully identified. At least 15 proteins were identified that are involved in host defense. These results demonstrate that the proteome of milk is more complex than has previously been reported and a significant fraction of minor milk proteins are involved in protection against infection.

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Eight primiparous cows in midlactation were used to determine a method for the mammary biopsy of standing cows in full lactation. Cows were mildly sedated; therefore, preoperative feed withdrawal was not necessary. A core of secretory tissue (0.75 to 1 g) was extracted using a rotating stainless steel cannula with a retractable blade at the cutting edge. Postoperative recovery was rapid, taking only 15 min per cow, and the method was reliable and efficient. The presence of secretory tissue was verified by histology and in situ hybridization with alpha s1-casein and alpha-lactalbumin probes. The capsular end of the core contained more connective tissue, and the parenchyma showed heterogeneous expression of alpha s1-casein and alpha-lactalbumin. Despite some postoperative bleeding, milk yield and composition in the biopsied gland were affected only transiently. Yield recovered by 3.5 d after biopsy, and composition recovered by 6.5 d after biopsy. Yield and composition of milk from the control glands were not affected by the procedure. Biopsy sites healed rapidly and without infection. No clinical mastitis was observed in any of the biopsied cows throughout the remainder of the lactation.