system in human milk remain controversial. Although some studies suggest that malnutrition may decrease the production or secretion of some of the components of the immunologic system in human milk, further investigations are needed to characterize more precisely the nutritional status of the mothers and the daily secretion of the immunologic factors.
In addition to the soluble immunologic agents mentioned above, human milk contains living white blood cells (leukocytes) (Crago et al., 1979; Smith and Goldman, 1968). Neutrophils and macrophages account for approximately 90% of the white blood cells in human milk; the remaining white blood cells are lymphocytes. The neutrophils have phagocytic activity and intracellular killing power similar to those of neutrophils in human blood (Ho and Lawton, 1978; Robinson et al., 1978; Smith and Goldman, 1968; Tsuda et al., 1984) and the bactericidal power of these cells appears to be spared in malnourished women (Bhaskaram and Reddy, 1981). However, the neutrophils in milk are less motile than their counterparts in blood. Moreover, unlike blood neutrophils, they do not appear to increase many of their functions in response to bacteria or serum-derived chemotactic agents (Thorpe et al., 1986).
The morphology of human milk macrophages suggests that they are activated; indeed, that is born out by the fact that they are more motile than their precursors in blood are (Özkaragöz et al., 1988). The macrophages in human milk are involved in antigen processing and presentation to T lymphocytes and thus may serve in the recognition of foreign materials. Furthermore, these macrophages display class II major histocompatibility antigens (Leyva-Cobián and Clemente, 1984), which suggests that they may participate in the process of immunogenesis in the infant.
Thymic-dependent lymphocytes (T cells) account for the majority of lymphocytes in milk; the relative proportions of the major subpopulations of these cells may be similar to those in blood (Keller et al., 1986). Although their cytotoxic capacities are poor, they can generate certain lymphokines when stimulated in vitro (Keller et al., 1981; Kohl et al., 1980; Lawton et al., 1979). The fate of these cells in the body is not known.
In the study of Narula et al. (1982) in poorly nourished Indian women whose malnutrition was defined by a body weight/height index, the total cell counts in milk collected on the second day of lactation were significantly lower than those found in well nourished Indian women. Cell counts obtained thereafter for as long as 180 days of lactation were similar in the well nourished and poorly nourished populations. Since appropriate cytochemical studies were not performed, it was difficult to determine whether any major alterations in cell populations in milk occurred as a result of changes in maternal nutritional status. More precise studies will be required to examine this question.