toxins to epithelial cells by acting as receptor analogs (Holmgren et al., 1981; Otnæss and Svennerholm, 1982; Otnæss et al., 1983).

Lipids

The hydrolysis of fats in human milk appears to generate fatty acids and monoglycerides with antiviral properties (Isaacs et al., 1986; Welsh and May, 1979; Welsh et al., 1979). This process may be catalyzed by the infant's own lipases as well as by the action of bile salt-stimulated lipase from human milk in the digestive tract of the recipient infant. The action of the antiviral lipids may be limited to a few enteric pathogens such as Giardia lamblia (Gillin et al., 1983, 1985) or encapsulated coronaviruses (Resta et al., 1985).

Proteins

Many of the whey proteins in human milk have direct protective effects against infection. Lactoferrin, one of the dominant whey proteins in human milk throughout lactation (Table 6-5) (Butte et al., 1984a; Goldman et al., 1982, 1983a,b), inhibits the multiplication of siderophilic (iron-absorbing) bacteria by competing with these microorganisims for ferric iron (Bullen et al., 1978; Stephens et al., 1980). The features of lactoferrin in human milk that are responsible for its antimicrobial effect are as follows:

  • Approximately 80% is in the apo- (unconjugated) form (Fransson and Lönnerdal, 1980).

  • The protein is relatively resistant to proteolysis (Brines and Brock, 1983; Samson et al., 1980).

  • Lactoferrin appears to interact with several other host resistance factors in the inhibition or killing of bacterial pathogens.

  • Certain forms of lactoferrin that do not bind to iron may inhibit the replication of some viruses (Furmanski et al., 1989).

Antibodies are abundant in human milk throughout lactation; they are

TABLE 6-5 Concentrations of Immunologic Factors in Human Milk During Several Phases of Lactationa

 

Mean Concentration, mg/ml ± SD,b by Stage of Lactation

Factors

2–3 days

1 month

6 months

1 year

2 years

Lactoferrin

5.3 ± 12.9

1.9 ± 0.3

1.4 ± 0.4

1.0 ± 0.2

1.2 ± 0.1

Secretory IgAc

2 ± 2.5

1 ± 0.3

0.5 ± 0.1

1 ± 0.3

1.1 ± 0.2

Lysozyme

0.09 ± 0.03

0.02 ± 0.03

0.25 ± 0.12

0.2 ± 0.1

0.19 ± 0.03

a From Goldman and Goldblum (1989b) with permission.

b SD = Standard deviation.

c IgA = Immunoglobulin A.



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