Through the years, knowledge about the immune properties and effects of human milk has grown. A recommended comprehensive review by one of the pioneers in the field, Dr. Armand Goldman, appeared in Breastfeeding Medicine (2007).Below are the highlights of just some of many known immune properties and functions of human milk.
Human milk immunoglobulins
Human milk contains all of the different antibodies (M, A, D, G, E), but secretory immunoglobulin A (sIgA) is the most abundant. Milk-derived sIgA is a significant source of passively acquired immunity for the infant during the weeks before the endogenous production of sIgA occurs. During this time of reduced neonatal gut immune function, the infant has limited defense against ingested pathogens. Therefore, sIgA is an important protective factor against infection.
Assuming that the mother and her infant, who are closely associated, share common flora, the antigenic specificity of the mother’s sIgA in her milk is directed against the same antigens in the neonate. Maternal immunoglobulin A (IgA) antibodies derived from the gut and respiratory immune surveillance systems are transported via blood and lymphatic circulations to the mammary gland, ultimately to be extruded into her milk as sIgA. The packaging of IgA with a secretory component unique to the mammary gland protects the sIgA from stomach acids, allowing it to reach the small intestine intact.
Other immunologic properties of human milk
In addition to antibodies, human milk has numerous factors that can affect the intestinal microflora of the baby. These factors enhance the colonization of some bacteria while inhibiting the colonization by others. The immunologic components include lactoferrin, which binds to iron, thus making it unavailable to pathogenic bacteria; lysozyme, which enhances sIgA bactericidal activity against gram-negative organisms; oligosaccharides, which intercept bacteria and form harmless compounds that the baby excretes; milk lipids, which damage membranes of enveloped viruses; and mucins, which are present on the milk-fat globule membrane. Mucins adhere to bacteria and viruses and help eliminate them from the body. Interferon and fibronectin have antiviral activities and enhance lytic properties of milk leukocytes.
Our understanding of the interactional effect of these bioactive constituents, the impact of microbiota on gut function, and development (and role of human milk in that development) is just beginning to be understood. These constituents clearly have profound effects of the health status of individuals throughout life, particularly during infancy.
Human milk leukocytes
Macrophages comprise 40-60% of the cells in colostrum, with the remainder of cells primarily consisting of lymphocytes and polymorphonucleocytes. Extruded into the milk are rare mammary epithelial cells and the plasma membrane-bound lipid droplets referred to as milk-fat globules. By 7-10 days postpartum, with the transition from colostrum to mature milk, the percentage of macrophages then increases to 80-90% at a concentration of 104 -105 human milk macrophages per milliliter of milk. Milk leukocytes can tolerate extremes in pH, temperature, and osmolality. They have been shown to survive for as long as a week in baboons and lambs.
Passive immunity from mother to recipient breastfeeding infant
While awaiting endogenous maturation of the baby’s own immunologic systems, various immunologic and bioactive milk components act synergistically to provide a passive immunologic support system from the mother to her infant in the first days to months after birth. Ingested milk passively immunizes the neonate. Numerous studies have clearly documented this scenario and its clinical benefit, demonstrating decreased risk for gastrointestinal and respiratory infections, particularly during the first year of life.
Evidence is increasing that these immune and bioactive substances prime the neonatal GI and immune systems in their selective recognition of antigens and development of cellular signaling. This may explain the decreased risk of intestinal and respiratory allergy in children who have been breastfed and the lower-than-predicted risk of autoimmune diseases in the breastfed population. Direct effects are difficult to prove given the multifactorial nature of such diseases; however, when taken together, the data support the beneficial nature of human milk for the developing infant.
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