The Home as a Modulator of Milk Immunity: Association Between Domestic Factors and Immune Cell Populations in Human...

Background/Objectives: Human breast milk is a biologically active fluid. It contains immune cells, stem cells, epithelial cells, and lactocytes. These components may support infant development and immune defense. While milk composition is known to vary with physiological and nutritional factors, the impact of the home environment remains poorly understood. The aim of this study was to examine how selected conditions affect the cellular composition of breast milk. Methods: We conducted a cross-sectional study involving 49 lactating mothers of healthy infants under 6 months of age. Breast milk samples were analyzed using flow cytometry. We measured proportions of immune cells (CD3⁺, CD4⁺, CD8⁺, CD19⁺, and CD16/56⁺), hematopoietic stem cells (CD34⁺), mesenchymal stem cells (CD105⁺, CD73⁺, and CD44⁺), and lactocytes (CD326⁺ CD73⁺ and CD326⁺ CD73^- phenotypes). Participants completed a questionnaire assessing number of children, co-sleeping, pet ownership, and number of household members. Results: Mothers with more than one child showed higher percentages of CD4⁺ (p = 0.047) and CD8⁺ (p = 0.031) T cells and fewer CD73⁺ lactocytes (p = 0.028). Co-sleeping was associated with lower levels of CD3⁺ T cells in milk (p = 0.021). Pet ownership correlated with a lower proportion of cytotoxic CD8⁺ cells (p = 0.048). The number of household members had no significant effect. Conclusions: Domestic factors such as number of children, co-sleeping, and pet exposure are associated with shifts in the immune and lactocyte cell composition of breast milk. These findings suggest that breast milk dynamically adapts to maternal and household-level immune stimuli.