IMMUNOLOGIC PROPERTIES OF HUMAN MILK
Category: Child Health
Abstract : IMMUNOLOGIC PROPERTIES 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 gu
IMMUNOLOGIC PROPERTIES 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 that
are enhanced by ingested breast milk.
Assuming that the mother and her
infant, who are closely associated, share common flora, the antigenic
specificity of the mother's milk sIgA will be 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, to be finally extruded into her
milk as sIgA. The packaging of IgA with a secretory component unique to the
mammary gland affords the sIgA protection from stomach acids to later reach the
small intestine intact.
Other immunological properties of human
milk
In addition to antibodies, human milk also has numerous factors that can
affect the intestinal microflora of the baby by enhancing the colonization of
some bacteria while inhibiting the colonization by others. These immunologic
components include lactoferrin, which binds to iron making it unavailable to
pathogenic bacteria; lysozyme, which enhances sIgA bactericidal activity against
gram-negative organisms; oligosaccharides, which intercept bacteria forming
harmless compounds that the baby excretes; milk lipids, which damage membranes
of enveloped viruses; and mucins, which 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.
Human milk leukocytes
Macrophages
comprise 40-60% of the cells in colostrum, with the remainder of cells
consisting primarily 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, and they have been shown to survive during the first postnatal
week in baboons and lambs.
Passive immunity from mother to her recipient
breastfeeding infant While awaiting endogenous maturation of the baby's own
immunologic systems, a variety of immunologic and bioactive milk components act
synergistically to provide a passive immunological support system extending from
the mother to her infant in the first days to months after birth. The neonate
becomes passively immunized through ingested milk. This scenario and its
clinical benefit are clearly documented by numerous studies that demonstrate a
decreased risk to gastrointestinal and respiratory infections, particularly
during the first year of life.
Evidence is increasing that these immune
and bioactive substances prime the neonatal gastrointestinal and immune systems
in their selective recognition of antigens and development of cellular
signaling. This premise 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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