Human Milk

Milk immunity and reproductive status among Ariaal women of northern Kenya

Abstract Background: The immune factors in human milk protect infants from infection and promote immune development. Evidence suggests that the production of milk-specific immunoproteins is energetically costly, making them a form of maternal investment in offspring health that is subject to life history trade-offs.

AIM

This research tests the relationship between measures of reproductive effort and the production of milk immunoglobulin A (IgA) among Ariaal women, a settled northern Kenyan pastoralist population that experiences nutritional and disease stress.

SUBJECTS AND METHODS

Two hundred and forty-three lactating Ariaal women participated in a questionnaire, anthropometric measurement and milk collection. Milk IgA was analysed using ELISA.

RESULTS

There was a significant U-shaped relationship between post-partum months and milk IgA, reflecting decreasing investment in the current offspring over the early post-partum period and subsequent recovery of maternal energy status near the end. There was also a significant inverse U-shaped relationship between milk IgA and parity, with high parity women showing a drop in milk IgA. In contrast, anthropometric measures of energy status were not associated with the production of milk IgA.

CONCLUSION

This research shows that patterns of reproductive investment can be demonstrated in milk IgA in the absence of measured energy changes.

Field and laboratory methods in human milk research

Human milk is a complex and variable fluid of increasing interest to human biologists who study nutrition and health. The collection and analysis of human milk poses many practical and ethical challenges to field workers, who must balance both appropriate methodology with the needs of participating mothers and infants and logistical challenges to collection and analysis. In this review, we address various collection methods, volume measurements, and ethical considerations and make recommendations for field researchers. We also review frequently used methods for the analysis of fat, protein, sugars/lactose, and specific biomarkers in human milk. Finally, we address new technologies in human milk research, the MIRIS Human Milk Analyzer and dried milk spots, which will improve the ability of human biologists and anthropologists to study human milk in field settings.

The roles of the immune system in women's reproduction: evolutionary constraints and life history trade-offs

Life history theory posits that, as long as survival is assured, finite resources are available for reproduction, maintenance, and growth/storage. To maximize lifetime reproductive success, resources are subject to trade-offs both within individuals and between current and future investment. For women, reproducing is costly and time-consuming; the bulk of available resources must be allocated to reproduction at the expense of more flexible systems like immune function. When reproducing women contract infectious diseases, the resources required for immune activation can fundamentally shift the patterns of resource allocation. Adding to the complexity of the reproductive-immune trade-offs in women are the pleiotropic effects of many immune factors, which were modified to serve key roles in mammalian reproduction. In this review, we explore the complex intersections between immune function and female reproduction to situate proximate immunological processes within a life history framework. After a brief overview of the immune system, we discuss some important physiological roles of immune factors in women's reproduction and the conflicts that may arise when these factors must play dual roles. We then discuss the influence of reproductive-immune trade-offs on the patterning of lifetime reproductive success: (1) the effect of immune activation/infectious disease on the timing of life history events; (2) the role of the immune system, immune activation, and infectious disease on resource allocation within individual reproductive events, particularly pregnancy; and (3) the role of the immune system in shaping the offspring's patterns of future life history trade-offs. We close with a discussion of future directions in reproductive immunology for anthropologists.

Mucosal immune system in neonatal period and early infancy

The major mechanisms of mucosal defense in the neonate consist of a variety of nonspecific barriers, and innate and specific adaptive immune responses. The functions of innate immunity in the mucosal surfaces are mediated by host-specific microbial–pathogen recognition receptors, designed to recognize unique microbial-associated molecular patterns integral to the structure of most microorganisms. Other mechanisms include many antimicrobial peptides, macrophages, dendritic cells, complement components and host-derived cellular and soluble products. The important elements of neonatal mucosal adaptive immunity include the inductive sites, such as the Peyer’s patches and other lymphoid structures in the respiratory and gastrointestinal tracts, the nasopharyngeal and sublingual tissues, the subepithelial and intraepithelial sites in most external mucosal surfaces, including the genital tract and the mammary glands. These sites contain lymphoid cells derived by the homing of antigen-activated cells from the inductive sites. Activated B cells, mostly IgA (up to 80%) are detected in the mucosal tissues shortly after birth. However, IgA-producing plasma cells are generally detected in the mucosa approximately 7–10 days of age. With progressive environmental antigenic stimulation, the number of circulating IgA cells increase significantly by 1 month of age. The postnatal development of mucosal immunity is critically influenced by the acquisition and nature of mucosal microflora, and the temporal nature and qualitative and quantitative aspects of dietary antigens and other environmental agents introduced in the neonatal period. Mucosal immune responses are generally protective against disease-producing organisms and environmental macromolecules. The mucosal immune responses may also be pathologic and foster the induction of immunologically mediated disease states and autoimmunity. The development of secretory IgA and other antimicrobial mucosal responses as well as the induction of tolerance in the neonatal period and early infancy are essential for the maintenance of mucosal homeostasis in early childhood and prevention of disease later in life.

Antiinfective properties of human milk

The unfavorable effects of neonatal immunodeficiency are limited by some naturally occurring compensatory mechanisms, such as the introduction of protective and immunological components of human milk in the infant. Breast-feeding maintains the maternal-fetal immunological link after birth, may favor the transmission of immunocompetence from the mother to her infant, and is considered an important contributory factor to the neonatal immune defense system during a delicate and crucial period for immune development. Several studies have reported that breast-feeding, because of the antimicrobial activity against several viruses, bacteria, and protozoa, may reduce the incidence of infection in infants. The protection from infections may be ensured either passively by factors with antiinfective, hormonal, enzymatic, trophic, and bioactive activity present in breast milk, or through a modulator effect on the neonatal immune system exerted by cells, cytokines, and other immune agents in human milk.