Immunologic Factors in Human Breast Milk
For the fetus and newborn, immunologic defenses are present,
but immature. To compensate, the mother’s immunoglobulin
(Ig) G antibody moves across the placental barrier to provide
some protection. After birth, these maternal antibodies
wane in the first 6 to 12 months of human life. The neonate and
infant can receive additional maternal protection from breast
milk, however.
Human breast milk contains large quantities of secretory
Ig A (sIgA). These antibodies, which have formed as a consequence
of the mother’s previous exposure to infectious agents,
can bind to potential pathogens and prevent their attachment
to the infant’s cells. Secretory IgA is adapted to survive in the
respiratory and gastrointestinal mucosal membranes and resist
proteolytic digestion. Secretory IgA neutralizes infectious
agents while at the same time limiting the damaging effects of
tissue inflammation that can occur with other antibody types.
Human breast milk, and especially the early colostrum,
contains measurable levels of leukocytes. Colostrum contains
approximately 5106 cells per mL, an amount that decreases
tenfold in mature milk. Most of these leukocytes are
macrophages and neutrophils, which phagocytose microbial
pathogens. Lymphocytes, including T cells, natural killer cells,
and antibody-producing B cells, make up 10% of the leukocytes
in human breast milk. There is evidence to suggest that these
cells survive passage through the infant’s gastrointestinal
system where they are absorbed and influence the infant’s
immune response.11 Much of this evidence comes from
animal studies, however, which will be discussed later in the
present review.
In addition to these immunologic components, breast
milk contains several nonspecific factors that have antimicrobial
effects.12 These factors include the enzyme
lysozyme, which inhibits the growth of many bacterial
species by disrupting the proteoglycan layer of the bacterial
cell wall. Lactoferrin, one of the most abundant proteins in
human milk, also limits bacterial growth by removing
essential iron. Nucleotides in human milk have been
shown to enhance immune function in infants.13 Complex
sugars are found only in trace amounts in cow milk
but make up a substantial portion of human milk sugars,
where they may prevent adherence of various microbial
pathogens by acting as decoy receptors.11