Breastfeeding is thought to play a major role in protecting infants from disease. However, the immunological effects of breastmilk have not been studied systematically within the context of biological anthropology. In order to understand the factors that influence the production and transfer of breastmilk immunity, I used an evolutionary and biocultural framework to assess the complex interplay between immunity, reproduction, growth, and culture in Ariaal mothers and infants of northern Kenya. Specifically, I investigated two research questions: (1) How do maternal reproductive and nutritional characteristics influence immune biomarkers in breastmilk? and (2) How do breastmilk immune factors protect infants from disease and affect their health and growth?

This dissertation utilized immunoglobulin A as a marker of immunity. It is found in abundance in breastmilk, providing pathogen protection to developing infants. Two hundred and fifty-one Ariaal mother-infant pairs were recruited into the study and gave breastmilk and saliva samples, underwent anthropometric measurement, and answered an extensive questionnaire. Immunoglobulin A was analyzed in breastmilk and saliva using a method I developed in the Clinical Ligand Assay Satellite Service laboratory at the University of Michigan.

This work yielded three main results. First, maternal reproductive status and history was associated with the amount of breastmilk immunoglobulin A transferred to the infant, although maternal nutritional status was not. Specifically, energetic preparation for future offspring and reproductive history influenced maternal ability to produce immunoglobulin A in breastmilk. Second, the level of immunoglobulin A in breastmilk was not associated with infant growth, health, or protection from disease-exposing behaviors, although it did appear to vary with changes in infant body fat, indicating that breastmilk immunoglobulin A and infant body fat may work symbiotically to protect infants from disease. Finally, stunted infants had greater activation of salivary immunoglobulin A than infants who were not stunted, suggesting that costly immune activation may trade-off with growth in infancy.

The results of this work indicate that reproductive and nutritional factors are intimately associated with the breastmilk immunoglobulin A system. Future long-term and comparative studies will assess the impact of immunoglobulin A in mothers and infants in other populations.