The effects of the duration of lactation upon lactoferrin, lysozyme, total IgA, SIgA,
SIgA antibodies to Escherichia coli somatic antigens and leukocytes in human milk
were investigated. Longitudinal and cross-sectional studies were performed with milk
collected from women 20 to 35 years of age during te first year of lactation. Collection
and storage conditions and immunologic analyses were controlled to minimize confounding
variables. The concentrations of lactoferrin, total IgA, and leukocytes and the uptake
of 3H-thymidine by phytohemagglutinin-stimulated lymphocytes fell during the first
several weeks of lactation; afterward, the levels of lactoferrin and IgA stabilized.
Approximately 90% of total IgA in human milk during the year was SIgA. Secretory IgA
antibody titers to E. coli increased in some individuals studied longitudinally suggesting
that the enteromammary gland pathway of SIgA antibody production was active after
several weeks of lactation. Moreover, the concentrations of lysozyme, after falling
to a nadir of 20 to 30 micrograms/ml at 2 to 4 weeks, rose to 200 to 300 micrograms/ml
by six months and remained elevated. The immunologic system in human milk undergoes
remarkable changes which may represent adaptations for the recipient infant.