Introduction and objective: The relationship between the bioactive components of milk and maternal and child health is not well elucidated yet. The aim of this study was to determine whether maternal factors (diseases and drugs used during pregnancy, method of delivery, age) and child characteristics (age, length and body weight at birth) affect breast milk composition (leukocytes, stem cells, and epithelial cells). Materials and methods: Twenty-three breastfeeding mothers of infants aged 1–6 months were recruited for the study. All the mothers and children were healthy, with no symptoms of respiratory tract infection. Samples of breast milk (20 mL) were collected. Each sample was assessed for its cellular composition (epithelial cells, mesenchymal stem cells, leukocytes). Only a few studies available in the literature have evaluated both immune cells (leukocytes) and cells of non-immune origin (mesenchymal and epithelial cells) in human milk. In this regard, our analysis is innovative. Results: Women receiving pharmacotherapy during pregnancy had a significantly higher percentage of eosinophils in their breast milk compared to women who did not receive any medication when pregnant. The cellular composition of milk correlated inversely with the age of the mother and child. The more advanced the mother’s age, the lower the number of monocytes in the milk. On the other hand, the older the child, the lower CD4 lymphocyte count in milk. Conclusions: The study suggests that the cellular composition of breast milk is influenced not only by the characteristics related to the mother but also by child-depended factors. Research in this area needs to be continued.