This study (Hassiotou, F., Hepworth, A. R., Metzger, P., Lai, C. T., Trengove, N., Hartmann, P. E., & Filgueira, L. (2013)) looks into how infants may signal to the mother that they require certain immune cells to fight pathogens. It was found that when a child is infected, the mother has more leukocytes present in her breast milk. The authors explore a phenomenon dubbed “backwash”, in which the baby’s saliva may go into the mother’s nipple and signal that it needs immune cells to fight a certain disease. The authors found that some milk flowed backward into the mother’s nipple, and hypothesized that through this, babies can order leukocytes; this works with passive immunity to make the child as safe as possible while he/she develops immunity.