How Breast Milk
Protects Newborns

Some of the molecules and cells
in human milk actively help
infants stave off infection

by Jack Newman

Doctors have long known that infants who are breast-fed contract fewer
infections than do those who are given formula. Until fairly recently, most
physicians presumed that breast-fed children fared better simply because milk
supplied directly from the breast is free of bacteria. Formula, which must
often be mixed with water and placed in bottles, can become contaminated
easily. Yet even infants who receive sterilized formula suffer from more
meningitis and infection of the gut, ear, respiratory tract and urinary tract
than do breast-fed youngsters.

The reason, it turns out, is that mother's milk actively helps newborns avoid
disease in a variety of ways. Such assistance is particularly beneficial
during the first few months of life, when an infant often cannot mount an
effective immune response against foreign organisms. And although it is not
the norm in most industrial cultures, UNICEF and the World Health
Organization both advise breast-feeding to "two years and beyond." Indeed, a
child's immune response does not reach its full strength until age five or
so.

All human babies receive some coverage in advance of birth. During pregnancy,
the mother passes antibodies to her fetus through the placenta. These
proteins circulate in the infant's blood for weeks to months after birth,
neutralizing microbes or marking them for destruction by phagocytes-immune
cells that consume and break down bacteria, viruses and cellular debris. But
breast-fed infants gain extra protection from antibodies, other proteins and
immune cells in human milk.

Once ingested, these molecules and cells help to prevent microorganisms from
penetrating the body's tissues. Some of the molecules bind to microbes in the
hollow space (lumen) of the gastrointestinal tract. In this way, they block
microbes from attaching to and crossing through the mucosa-the layer of
cells, also known as the epithelium, that lines the digestive tract and other
body cavities. Other molecules lessen the supply of particular minerals and
vitamins that harmful bacteria need to survive in the digestive tract.
Certain immune cells in human milk are phagocytes that attack microbes
directly. Another set produces chemicals that invigorate the infant's own
immune response.

Breast Milk Antibodies

Antibodies, which are also called immunoglobulins, take five basic forms,
denoted as IgG, IgA, IgM, IgD and IgE. All have been found in human milk, but
by far the most abundant type is IgA, specifically the form known as
secretory IgA, which is found in great amounts throughout the gut and
respiratory system of adults. These antibodies consist of two joined IgA
molecules and a so-called secretory component that seems to shield the
antibody molecules from being degraded by the gastric acid and digestive
enzymes in the stomach and intestines. Infants who are bottle-fed have few
means for battling ingested pathogens until they begin making secretory IgA
on their own, often several weeks or even months after birth.

The secretory IgA molecules passed to the suckling child are helpful in ways
that go beyond their ability to bind to microorganisms and keep them away
from the body's tissues. First, the collection of antibodies transmitted to
an infant is highly targeted against pathogens in that child's immediate
surroundings. The mother synthesizes antibodies when she ingests, inhales or
otherwise comes in contact with a disease-causing agent. Each antibody she
makes is specific to that agent; that is, it binds to a single protein, or
antigen, on the agent and will not waste time attacking irrelevant
substances. Because the mother makes antibodies only to pathogens in her
environment, the baby receives the protection it most needs-against the
infectious agents it is most likely to encounter in the first weeks of life.

Second, the antibodies delivered to the infant ignore useful bacteria
normally found in the gut. This flora serves to crowd out the growth of
harmful organisms, thus providing another measure of resistance. Researchers
do not yet know how the mother's immune system knows to make antibodies
against only pathogenic and not normal bacteria, but whatever the process may
be, it favors the establishment of "good bacteria" in a baby's gut.

Secretory IgA molecules further keep an infant from harm in that, unlike most
other antibodies, they ward off disease without causing inflammation-a
process in which various chemicals destroy microbes but potentially hurt
healthy tissue. In an infant's developing gut, the mucosal membrane is
extremely delicate, and an excess of these chemicals can do considerable
damage.
Interestingly, secretory IgA can probably protect mucosal surfaces other than
those in the gut. In many countries, particularly in the Middle East, western
South America and northern Africa, women put milk in their infants' eyes to
treat infections there. I do not know if this remedy has ever been tested
scientifically, but there are theoretical reasons to believe it would work.
It probably does work at least some of the time, or the practice would have
died out.

An Abundance of Helpful Molecules

Several molecules in human milk
besides secretory IgA prevent microbes from attaching to mucosal surfaces.
Oligosaccharides, which are simple chains of sugars, often contain domains
that resemble the binding sites through which bacteria gain entry into the
cells lining the intestinal tract. Thus, these sugars can intercept bacteria,
forming harmless complexes that the baby excretes. In addition, human milk
contains large molecules called mucins that include a great deal of protein
and carbohydrate. They, too, are capable of adhering to bacteria and viruses
and eliminating them from the body.

The molecules in milk have other valuable functions as well. Each molecule of
a protein called lactoferrin, for example, can bind to two atoms of iron.
Because many pathogenic bacteria thrive on iron, lactoferrin halts their
spread by making iron unavailable. It is especially effective at stalling the
proliferation of organisms that often cause serious illness in infants,
including Staphylococcus aureus. Lactoferrin also disrupts the process by
which bacteria digest carbohydrates, further limiting their growth.
Similarly, B12 binding protein, as its name suggests, deprives microorganisms
of vitamin B12.
Bifidus factor, one of the oldest known disease-resistance factors in human
milk, promotes the growth of a beneficial organism named Lactobacillus
bifidus. Free fatty acids present in milk can damage the membranes of
enveloped viruses, such as the chicken pox virus, which are packets of
genetic material encased in protein shells. Interferon, found particularly in
colostrum-the scant, sometimes yellowish milk a mother produces during the
first few days after birth-also has strong antiviral activity. And
fibronectin, present in large quantities in colostrum, can make certain
phagocytes more aggressive so that they will ingest microbes even when the
microbes have not been tagged by an antibody. Like secretory IgA, fibronectin
minimizes inflammation; it also seems to aid in repairing tissue damaged by
inflammation.

Cellular Defenses

As is true of defensive molecules, immune cells are abundant in human milk.
They consist of white blood cells, or leukocytes, that fight infection
themselves and activate other defense mechanisms. The most impressive amount
is found in colostrum. Most of the cells are neutrophils, a type of phagocyte
that normally circulates in the bloodstream. Some evidence suggests that
neutrophils continue to act as phagocytes in the infant's gut. Yet they are
less aggressive than blood neutrophils and virtually disappear from breast
milk six weeks after birth. So perhaps they serve some other function, such
as protecting the breast from infection.

The next most common milk leukocyte is the macrophage, which is phagocytic
like neutrophils and performs a number of other protective functions.
Macrophages make up some 40 percent of all the leukocytes in colostrum. They
are far more active than milk neutrophils, and recent experiments suggest
that they are more motile than are their counterparts in blood. Aside from
being phagocytic, the macrophages in breast milk manufacture lysozyme,
increasing its amount in the infant's gastrointestinal tract. Lysozyme is an
enzyme that destroys bacteria by disrupting their cell walls.

In addition, macrophages in the digestive tract can rally lymphocytes into
action against invaders. Lymphocytes constitute the remaining 10 percent of
white cells in the milk. About 20 percent of these cells are B lymphocytes,
which give rise to antibodies; the rest are T lymphocytes, which kill
infected cells directly or send out chemical messages that mobilize still
other components of the immune system. Milk lymphocytes seem to behave
differently from blood lymphocytes. Those in milk, for example, proliferate
in the presence of Escherichia coli, a bacterium that can cause
life-threatening illness in babies, but they are far less responsive than
blood lymphocytes to agents posing less threat to infants. Milk lymphocytes
also manufacture several chemicals-including gamma-interferon, migration
inhibition factor and monocyte chemotactic factor-that can strengthen an
infant's own immune response.

Added Benefits

Several studies indicate that some factors in human milk may induce an
infant's immune system to mature more quickly than it would were the child
fed artificially. For example, breast-fed babies produce higher levels of
antibodies in response to immunizations. Also, certain hormones in milk (such
as cortisol) and smaller proteins (including epidermal growth factor, nerve
growth factor, insulinlike growth factor and somatomedin C) act to close up
the leaky mucosal lining of the newborn, making it relatively impermeable to
unwanted pathogens and other potentially harmful agents. Indeed, animal
studies have demonstrated that postnatal development of the intestine occurs
faster in animals fed their mother's milk. And animals that also receive
colostrum, containing the highest concentrations of epidermal growth factor,
mature even more rapidly.

Other unknown compounds in human milk must stimulate a baby's own production
of secretory IgA, lactoferrin and lysozyme. All three molecules are found in
larger amounts in the urine of breast-fed babies than in that of bottle-fed
babies. Yet breast-fed babies cannot absorb these molecules from human milk
into their gut. It would appear that the molecules must be produced in the
mucosa of the youngsters' urinary tract. In other words, it seems that
breast-feeding induces local immunity in the urinary tract.

In support of this notion, recent clinical studies have demonstrated that the
breast-fed infant has a lower risk of acquiring urinary tract infections.
Finally, some evidence also suggests that an unknown factor in human milk may
cause breast-fed infants to produce more fibronectin on their own than do
bottle-fed babies.

All things considered, breast milk is truly a fascinating fluid that supplies
infants with far more than nutrition. It protects them against infection
until they can protect themselves.

The Author

JACK NEWMAN founded the breast-feeding clinic at the Hospital for Sick
Children in Toronto in 1984 and serves as its director. He has more recently
established similar clinics at Doctors Hospital and St. Michael's Hospital,
both in Toronto. Newman received his medical degree in 1970 from the
University of Toronto, where he is now an assistant professor. He completed
his postgraduate training in New Zealand and Canada. As a consultant for
UNICEF, he has worked with pediatricians in Africa. He has also practiced in
New Zealand and in Central and South America.

Further Reading

MUCOSAL IMMUNITY: THE IMMUNOLOGY OF BREAST MILK. H. B. Slade and S. A.
Schwartz in Journal of Allergy and Clinical Immunology, Vol. 80, No. 3, pages
348-356; September 1987.

IMMUNOLOGY OF MILK AND THE NEONATE. Edited by J. Mestecky et al. Plenum
Press, 1991.

BREASTFEEDING AND HEALTH IN THE 1980'S: A GLOBAL EPIDEMIOLOGIC REVIEW. Allan
S. Cunningham in Journal of Pediatrics, Vol. 118, No. 5, pages 659-666; May
1991.

THE IMMUNE SYSTEM OF HUMAN MILK: ANTIMICROBIAL, ANTIINFLAMMATORY AND
IM-MUNOMODULATING PROPERTIES. A. S. Goldman in Pediatric Infectious Disease
Journal, Vol. 12, No. 8, pages 664-671; August 1993.

HOST-RESISTANCE FACTORS AND IMMUNOLOGIC SIGNIFICANCE OF HUMAN MILK. In
Breastfeeding: A Guide for the Medical Profession, by Ruth A. Lawrence. Mosby
Year Book, 1994.

SCIENTIFIC AMERICAN  December 1995  Volume 273  Number 6  Page 76


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