Immunological Protection

This article was originally written in 1999 by Kathryn Orlinsky.

The immunological properties of human milk may be as important as the nutritional aspects of human milk. How do we know that human milk really does make a difference in preventing disease? Many studies show that children who are fed human milk substitutes end up sick or hospitalized at a much greater rate than children who breastfeed. For example, diarrhea was significantly reduced in infants fed human milk compared with bottle-fed and weaned children.^9,19,20 Most bacterial infections are caused by organisms that are already colonizing the host, usually in the gastrointestinal or respiratory tracts. Human milk may prevent the growth of these bacteria.

There are several ways in which human milk protects children from infection. Human milk contains antibodies (immunoglobulins) which can help ward off disease in the infant. Human milk is particularly high in immunoglobulin A (IgA);¹⁰ there is more IgA in human milk than in serum.⁶ IgA binds to viruses and bacteria, particularly those that enter through the gut and other mucus membranes. This is especially protective of infants, who are always putting things in their mouths.

In addition to antibodies, human milk also contains lysozyme and lactoferrin, two antibacterial enzymes which protect against a host of infectious agents, including E. coli and Staphylococcus.³

Human milk also contains whole immune cells called white blood cells. Many of these cells are phagocytes, so called because they engulf bacteria and viruses, especially if these germs have IgA attached to them. Other immune cells in human milk include B cells, which make antibodies, and T cells, which attack diseased cells.¹⁷

How can these antibodies and other proteins help the infant if they are swallowed and digested along with the nutritional components of human milk? It turns out that these immune factors are resistant to proteolysis in the infant's gut.^7,8 Ninety percent of the IgA in human milk exists in a complex with secretory component (SIgA).¹⁰ SIgA is resistant to trypsin digestion in the neonatal gut.²²

Another way human milk protects children from disease is by favoring the growth of beneficial microorganisms. These benign colonizations prevent dangerous infections from taking hold. For example, bifidobacteria are more numerous in breastfed infants than in those fed with human milk substitutes.^1,3 This may be because of nucleotide salts present only in human milk, however, when these nucleotide salts were added to the human milk substitutes, the growth of bifidobacteria was still discouraged.¹ Breastfeeding may also favor the proliferation of bacteria with decreased virulence.^15,21 These strains may be more sensitive to bactericidal agents in serum, more prone to agglutinization, or less able to attach to epithelial surfaces.¹⁴

Human milk does not contain the same proportion of immune cells and antibodies that are found in the mother's blood, nor does it contain a stationary amount of these protective agents.⁴ Human milk contains much more IgA and less immunoglobulin M than serum does.⁶ In addition, the type of T cells predominantly found in human milk is different from the predominant type of T cell found in serum.¹⁷ This is because the mammary glands themselves contain lymphoid cells which produce the IgA antibodies needed by the breastfeeding child.^10,13,18 This mechanism is functional throughout lactation. In addition to providing protection from specific germs in the infant, the production and secretion of these immunological factors by the mother's mammary gland may be linked to the development of the child's own immune system.⁴

Skeptics have said, "yes, human milk benefits infants, but older children cannot continue to receive immunity by breastfeeding, can they?" The answer to this question is a resounding yes. Children enjoy health benefits for as long as they breastfeed. Studies have compared weaned children with breastfeeding children at 30 months¹⁶ and at 36 months,^20,23 and found them to be sicker. In some parts of the world, weaned toddlers have a mortality rate three and a half times higher than toddlers who receive human milk.²⁰ Weaning foods and even water from some regions are highly contaminated with E. coli,² but even undernourished mothers from these regions produce ample milk antibodies.⁵

There are at least two reasons why breastfeeding continues to benefit older children. First, human milk contains immune factors regardless of the duration of lactation. Both lysozyme and SIgA levels have been found in human milk for the entire period of lactation studied, including the second year.^10,12,13 Many of these immune factors would be otherwise unavailable.¹³ Second, human milk is more easily tolerated by a sick child than weaning foods. Thus, breastfeeding ensures that sick children remain hydrated and do not lose excessive weight. For a more detailed description of how breastfeeding can help an older child combat a severe illness, see What if he hadn't been nursing?

There are special cases where human milk is particularly high in immune factors. Colostrum is exceptionally rich in immune factors, containing more white blood cells per unit volume than blood.¹⁷ Preterm milk also contains more immune factors than term milk, both in colostrum and mature milk.¹¹ Newborns and premature infants need immunological protection more than toddlers, and they get that increased protection in the human milk they consume. This does not preclude older children from benefiting immunologically from continuing to breastfeed.

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