The development of mammary secretory immunity in the human newborn.

Milk protein concentrations were determined either by radioimmunoassay (IgA and IgG) or single radial immunodiffusion (IgM, lactoferrin, albumin and lysozyme) in single or serial samples of neonatal milk (witch's milk) obtained from 33 healthy newborns (seven of whom were light for dates), four ill newborns following major surgery and four newborns suffering from one of a variety of infections. In addition, paired neonatal milk and heel prick blood samples were collected from seven newborns, and paired neonatal milk and maternal milk samples were collected from a further seven neonates and their respective mothers. The concentrations of secretory IgA (11S IgA) in neonatal milk, although 300-fold lower than the corresponding IgA concentrations in maternal milk, when compared with neonatal serum IgA concentrations, were consistent with local synthesis of IgA occurring in the neonatal mammary gland. Among the milk protein studies, on 11S IgA concentrations increased significantly after birth in neonatal milk, the rise being unrelated to the gestational age of the newborn. As an entero-mammary circulation of IgA precursor lymphocytes exists in adults, it is suggested that the entry of foreign antigens into the neonatal gut after birth may be an important factor influencing the development of neonatal secretory immunity.