INTRODUCTION

There are numerous wonderful reasons for breastfeeding a baby from both the mother's and baby's point of view. Many of these relate to satisfying the emotional needs of both. There are the obvious nutritional benefits. Then there are the issues of convenience and economy. Breastmilk is free. It is always on hand, always the right temperature and consistency; no need for mixing or sterilising bottles and teats. But if these reasons fail to convince you of the importance of breastfeeding then the overwhelming evidence that shows that breastfeeding is the single most important thing you can do to ensure your baby's health surely must.

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It has been known for a long time that babies who are breast-fed contract fewer infections than do those who are given formula. This is because breastmilk actively helps newborns avoid disease in a variety of ways. This is particularly important during the first few months of life, when an infant often cannot mount an effective immune response against foreign organisms1. Research shows a baby's immune system does not fully mature for several years1 , 2. While it is developing, the baby will be protected by being breastfed. In addition the baby's own immune system develops more rapidly than does baby who is fed formula.1 ,2 

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Over recent years there has been research undertaken into the factors present in human milk that protect infants from infection and illness. Early on researchers became aware that certain substances (most notably IgA, lysozyme, and lactoferrin) that are abundant in human milk3 might protect babies from infection.4 

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During pregnancy, antibodies are passed to the baby through the placenta. Dr Jack Newman says that "these proteins circulate in the infant's blood for weeks to months after birth, neutralising microbes or marking them for destruction by phagocytes - immune cells that consume and break down bacteria, viruses and cellular debris."1 However, when a baby is breastfed he or she is provided with extra protection from antibodies, other proteins and immune cells in human milk.

 

THE PROTECTIVE CONSTITUENTS OF BREASTMILK

Human milk contains proteins, non-protein nitrogen compounds, carbohydrates, lipids (fats), minerals and vitamins. Many of these components of breastmilk have non-nutritional roles some of which involve protecting the baby against infectious agents and disease.7  Human milk contains both specific immune factors (immunoglobulins or antibodies and certain cellular systems) and non-specific immune factors (lactoferrin, lysozyme, lactoperoxidase, complement, vitamin B12-binding protein, folic acid-binding protein, bifidus factor, lipid, interferon, cells).5 Specific immune factors have "adaptive memory that leads to the production of antibodies and certain cells (lymphocytes)" whereas non-specific immune factors have no intrinsic memory.5

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It is not the intention of this article to provide a comprehensive dissertation on the composition of human milk. What follows is a summary of some of the most important protective components of human milk and what protection they offer the breastfed infant.

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Antibodies

Breastfed babies receive antibodies - highly specific protection against disease.2, 6 New-born babies have very immature immune systems and little ability to fight illness-causing germs. Antibodies are made by the mother's immune system and are passed to the baby through the breast milk.1 

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Antibodies (also called immunoglobulins) take five basic forms: IgA, IgD, IgE, IgG and IgM.1, 7, 8 The most abundant is IgA, specifically the form known as secretory IgA, which is found in great amounts throughout the gut and respiratory system of adults. Babies do not make secretory IgA on their own for several weeks or even months after birth1  so the IgA that they receive through breastmilk is vital to the ability to fight ingested pathogens (disease causing micro-organisms such as bacteria and viruses). The antibodies passed on to the baby are antibodies made by the mother to fight pathogens in her environment; therefore the baby receives the protection it most needs against the pathogens it is most likely to encounter as a new-born.1, 2

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The way in which antibodies work are complex and the some of the processes are not entirely understood by researchers. For example the antibodies that the baby receives do not harm the good bacteria that colonise the intestines. These good bacteria "crowd out the growth of harmful organisms, thus providing another measure of resistance"1 . In addition, Newman when talking about secretory IgA states 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". He goes on to say that "secretory IgA can probably protect mucosal surfaces other than those in the gut" and notes that breast milk has been used by mothers' to treat eye infections. 

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Cellular Defenses 

Immune cells, white blood cells, or leukocytes, that fight infection themselves and activate other defence mechanisms, are abundant in human milk particularly in colostrum.1 , 7. In order of abundance these cells are neutrophils, macrophages and lymphocytes. Macrophages are cells that kill bacteria, fungi, and viruses. Macrophages also manufacture lysozyme, an enzyme that destroys bacteria by disrupting their cell walls.1, 5 Lymphocytes comprise B lymphocytes, which give rise to antibodies, and T lymphocytes, which kill infected cells directly or send out chemical messages that mobilise still other components of the immune system.1,7 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.1

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In addition, macrophages in the digestive tract can rally lymphocytes into action against invaders. Lymphocytes constitute 10 percent of white cells in the milk and 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.

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Lactoferrin

Lactoferrin is  a natural antibiotic and protects the new-born baby from bacteria that can multiply in its stomach and intestines and cause vomiting and diarrhoea. The concentration of lactoferrin is particularly high in colostrum and the amount of lactoferrin in the milk decreases over time as the danger from germs recedes and the baby's own defences develop. Lactoferrin prevents bacteria from getting any iron, the metal that is essential for germs (including Staphylococcus aureus1) to multiply rapidly, thus stopping them from getting out of control. In addition lactoferrin stops bacteria digesting carbohydrates, further limiting their growth.1 Lactoferrin can also move iron around the body to where it is needed.9

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Alpha-lactalbumin

Swedish and British immunologists have discovered that one identified protein in breast milk, alpha-lactalbumin, destroys cancer cells with which it comes into contact.10 According to lead scientist Catharina Svanborg, her team pursued this exciting research in an attempt to determine why "the relative risk of childhood lymphoma is nine times higher in bottle-fed infants, and the risk for carcinoma is also elevated. "Alpha-lactalbumin is a "substance that kills lots of tumor cells, every cancer we test it against. Lung cancer, throat cancer, kidney cancer, colon cancer, bladder cancer, lymphoma, leukemia, and pneumococcus bacteria too."11

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Other immune and non-immune protective agents

The main carbohydrate in human milk is lactose. Any lactose not absorbed by the infant reaches the large intestine where it is fermented to lactic acid. The acid conditions which result help promote the growth of Bifidobacteria and Lactobacilli, beneficial colonising bacteria, thus restricting colonisation by disease causing organisms. Certain nitrogen containing oligosaccharides have a growth promoting effect on Lactobacillus bifidus, the predominant gut flora of the breast-fed infant. These compounds are known as the 'bifidus factor'5 and are one of the oldest known disease-resistance factors in human milk. The bifidus factor is important in the prevention of diarrhoea in babies and is not found in cows milk.5

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Some oligosaccharides can also 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.1

Milk prostaglandins (found in the lipid - or milk fat - fraction5) are  present in fresh human milk and are believed to protect the immature and delicate gastrointestinal mucosa of new-born babies. This is supported by the fact that in premature babies, normally at risk from severe gastrointestinal illnesses, it is rare to see necrotising enterocolitis when they are breastfed.12

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Lactadherin is a glycoprotein which specifically attaches to rotaviruses and inhibits their settling on the gut wall. Newburg et al. reported on a study13 which showed that the protection that breastmilk provides against symptomatic rotavirus infection is associated with this lactadherin. 

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B12 binding protein deprives micro-organisms of vitamin B12. Interferon, found primarily in colostrum, has strong anti-viral activity and fibronectin, also present 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. Fibronectin minimizes inflammation and seems to aid in repairing tissue damaged by inflammation.1

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Recently, casein has been shown to prevent the attachment of Helicobacter pylori to human gastric mucosa.14

Some hormones in milk (such as cortisol) and smaller proteins (including epidermal growth factor, nerve growth factor, insulin-like growth factor and somatomedin C) act to close up the leaky mucosal lining of the young baby, making it relatively impermeable to unwanted pathogens and other potentially harmful agents.1

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Breastmilk Protects Babies from Illness

Numerous studies have shown that breastfeeding can protect babies from many illnesses, including ear infections, upper and lower respiratory tract infections, allergies, gastrointestinal disorders, colds and other  viruses, staphylococcus, streptococcus and E. coli infections, diabetes, juvenile rheumatoid arthritis, many childhood cancers, meningitis, pneumonia, urinary tract infections, salmonella, Sudden Infant Death Syndrome(SIDS).2, 8 Dr Margit Hamosh writes in Breast-feeding: Unraveling the Mysteries of Mother's Milk7 that "in addition to protection against some infectious diseases, breast-fed infants might also be protected at later ages from diseases that are sequelae of infectious insults." These include insulin-dependent diabetes mellitus,15 lymphoma,16 Crohn's disease,17  and coeliac disease.18 

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Dr John May, from La Trobe University has stated that "epidemiological studies have been important in demonstrating that breastfeeding clearly protects infants against respiratory and gastointestinal infections, or decreases the severity of these infections." He also says that "human milk contains a variety of potential anti-inflammatory agents, immunomodulators and bioactive compounds that may influence the incidence of diarrhoea in infected infants."8 Other researchers have confirmed that IgA antibodies play an important role in the protection of the newborn against enteropathogenic E. coli infections.19

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Other studies have concluded that breastfeeding results in a lower incidence of necrotizing enterocolitis20  and other gastrointestinal and respiratory infections in breast-fed infants than in formula-fed infants21 including infections caused by rotaviruses and respiratory syncytial viruses22. Golding et al. write that "prolonged breastfeeding, as well as providing protection against digestive tract infections, provided an important protection against Haemophilus influenzae B infections and against the occurrence of otitis media."23

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Several other studies have found that the incidence of otitis media is lower among breastfed babies7, 24 Duffy et al. reported in Pediatrics that the longer the duration of breastfeeding the lower the incidence of otitis media but that even for periods as short as three months, breastfeeding reduces the risk. They found that there was a two-fold risk of suffering a first episode of otitis media in children fed exclusively with formula, as opposed to those fed exclusively with breast milk for the first six months of life.

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In a study undertaken in 1992-93, Hylander et al. looked at the "effect of human milk on the incidence of infections in very low-birth-weight infants in the first stages of hospitalisation"25 The authors found that "the incidence of infections (human milk 29.3% vs. formula 47.2%) and sepsis/meningitis (human milk 19.5% vs. formula 32.6%) were significantly different according to the type of feeding." They concluded that very low-birth-weight infants who are fed with breast milk fared significantly better than those that were formula fed.

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Breast-feeding over time - how the protection changes

The concentration of various protective factors in human milk varies over time from the first few days when the infant receives colostrum through to the milk that a toddler receives. Dr Margit Hamosh says that the "daily production of many immuno-protective factors changes as lactation proceeds. The secretion of many soluble defense agents by the mammary gland is inversely related to the capacity of the recipient infant to produce them at mucosal sites."7

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Dr Hamosh7 writes at length on the changing character of breastmilk over time: "Colostrum contains higher concentrations of protein (including higher levels of protective proteins such as secretory IgA, lactoferrin, and lysozyme), sodium, and chloride, and contains lower amounts of potassium, carbohydrate, fat, and certain vitamins." She goes on to say that the changing concentration of immunoglobulins (IgA, IgG and IgM) illustrates the interaction between milk components and the functional development of the baby: while IgG and IgM rise rapidly after birth, the newborn maintains low levels of IgA during the first year of life. "Immune and non-immune protecting agents are present in milk throughout lactation and some, such as lysozyme, are present at higher concentrations during prolonged lactation than during the early stages."

 

The lessening concentration of some protective factors in human milk are compensated for by the increased volume of milk ingested by the baby over time. While the concentration of protective factors may decline, the disease-fighting properties of human milk do not disappear with the colostrum. As long as a baby receives breast milk he or she will receive immunological protection against many different viruses and bacteria2 . However, Wendy Oddy from the Telethon Institute for Child Health Research in Australia, says that there may prolonged protection against infections after the cessation of breastfeeding due to the impact of breast milk on the baby's developing immune system26.

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Antibacterial, Antiviral and Antiparasitic Factors in Human Milk

Research undertaken by the Department of Microbiology at the La Trobe University has shown that various antibacterial, antiviral and antiparasitic factors in human milk are active (in vitro) against a wide range disease causing organisms8 . Breastfed babies are protected by these factors which include immunoglobulins (Secretory IgA, IgG, IgM and IgD) Bifidobacterium bifidum growth factors, factor finding proteins, lactoferrin, lactoperoxidase, lysozyme, lipids and carbohydrates together with various other factors.

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Bacterial illnesses which these factors protect against include: E. coli , C. tetani, C. diphtheriae, K. pneumoniae, various  Staphylococci and Streptococci, H. influenzae. H. pylori, B. pertussis, Salmonella, Candida albicans, V. cholerae among others.

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Viral illnesses which these factors protect against include: Polio types, 1, 2 and 3, various Coxsackie viruses, rotavirus, rubella, varicella-zoster, rhinovirus, herpes simplex, mumps, influenza, respiratory syncytial virus, human immunodeficiency virus, hepatitis C, hepatitis B, measles, cytomegalovirus, Epstein-Barr, influenza among others.

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Parasites which these factors protect against include: Giardia lamblia, Schistosoma mansoni, Cryptosporidium, Toxoplasma, Plasmodium falciparum (malaria), Trichomonas vaginalis among others.

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1    Newman, Jack, (MD, FRCPC) 1995: How Breast Milk Protects New-borns, Scientific American, December 1995 Vol. 273 No. 6 Page 76.

2    La Leche League, 2001: on www.lalecheleague.com

3    Goldman, A.S. 1993: The immune system of human milk: Antimicrobial, antiinflammatory, and immunomodulating properties,. Pediatric Infectious Diseases Journal, 12:664-672.

4    Hamosh, M., Ellis, L.A., Pollock, D.R., et al., 1995: Breast-feeding and the working mother: Effect of time and temperature of short term storage on proteolysis, lipolysis and bacterial growth in milk. Pediatrics 97:492-498.

5    Rumball, S. and Darragh, A., 1995: Biology of Human Milk in Human Milk, Lactation and Infant Feeding Study Guide, Department of Chemistry and Biochemistry, Massey University.

6   Packard, V.S., 1982: Human Milk and Infant Formula, Academic Press, pg 72-747    

7   Hamosh, Margit, Ph.D.: Breast-feeding: Unraveling the Mysteries of Mother's Milk, at http://www.medscape. com/Medscape/WomensHealth/journal/1996/v01.n09/w120.hamosh/w120.hamosh.html

8   May, J.T., PhD, 2001: Molecular Virology: Tables of Antimicrobial Factors and Microbial Contaminants in Human Milk, Based on tables presented in the Proceedings of Breast Milk and Special Care Nurseries: Problems and Opportunities Conference. August 1995. Melbourne.

9    Emsley, John, 1988: The magic ingredient in a mother's milk, Kings College, London, published in The Independent

10     Hakansson, A., Zhivotovsky, B., Orrenius, S., Sabharwal, H., Svanborg, C., Apoptosis induced by a human milk protein, Proc Natl Acad Sci., 92 (17): 8064-8. 

11    Radetsky, P., 1999: Got cancer killers, Discover, June 1999.

12    Bedrick, A., 1990: The physiologic significance of Milk-borne prostaglandins for the developing gastrointestinal tract, Mammary Gland Biology and Lactation Newsletter, Vol. 9, No. 4

  13  Newburg, D.S., Peterson, J.A/, Ruiz Palacios, G.M., Matson, D,O,, Morrow, A.L., Shults J, et al. 1998: Role of human-milk lactadherin in protection against symptomatic rotavirus infection, The Lancet, 351(9110): 1160-4

14  Stromquist, M., Folk, P., Bergstrom, S., et al. 1995: Human milk k-casein and inhibition of Helicobacter pylori adhesion to human gastric mucosa, Journal of Pediatric Gastroenterology and Nutrition, 21: 288-296, 1995.

15   Mayer, E.J., Hamman, R.F., Gay, E.C. et al. 1988: Reduced risk of IDDM among breast fed children. The Colorado IDDM Registry, Diabetes 37(12):1625-1632.

16   Davis, M.K., Savitz, D.A. and Grauford, B.1988: Infant feeding and childhood cancer, The Lancet 2(8607):365-368.

17  Koletzko, S., Sherman, P., Corey, M. et al 1989: Role of infant feeding practices in development of Crohn's disease in childhood, British Medical Journal, 298(6688):1617-1618.

18  Challacombe, D.N., Mecrow, I.K., Elliot, K., Clarke, F.J. and Wheeler, E.E. 1997: Changing infant feeding practices and declining coeliac disease in West Somerset, Archives of Diseases in Childhood, 77(3): 206-9.

19 Carbonare, S.B., Silva, M.L., Palmeira, P. and Carneiro Sampaio, M.M., 1997: Human colostrum IgA antibodies reacting to enteropathogenic Escherichia coli antigens and their persistence in the feces of a breastfed infant, Journal of Diarrhoeal Disease Research, 15(2): 53-8.

20   Lucas, A. and Cole, T.J. 1990: Breast milk and neonatal necrotising enterocolitis, The Lancet 336 (8730): 1519-23.

21   Beaudry, M., Dufour, R. and Marcoux, S. 1995: Relation between infant feeding and infections during the first six months of life, Journal of Pediatrics, 126(2): 191-7.

22 Grover, M. et al. 1997: Effect of human milk prostaglandins and lactoferrin on respiratory syncytial virus and rotavirus" Acta Paediatrics, 86: 315-316.

23 Golding. J. Emmett, P.M. and Rogers, I.S., 1997:Does breast-feeding protect against non-gastric infections? Early Human Development, 49 Suppl: S105-20.

24  Duffy, L.C., Faden, H., Wasielewski, R., Wolf, J. and Krystofik, D., 1997: Exclusive breastfeeding protects against bacterial colonization and day care exposure to otitis media, Pediatrics, 100(4): E7.

25  Hylander, M.A., Strobino, D.M. and Dhanireddy, R. 1998: Human milk feedings and infection in very low birth weight infants, Pediatrics, 102(3): E38.

26  Oddy, W.H., 2001: Breastfeeding protects against illness and infection in infants and children: a review of the evidence, Breastfeed Rev., 9 (2): 11-8. 

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Copyright © 2002 Susan Claridge.

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