Serum IgA in the neonate. Molecular size, concentration and effect of breast feeding.

IgA concentrations in the serum of 48 six-day-old neonates (23 exclusively artificially fed, 25 exclusively breast fed) were measured using a double antibody radioimmunoassay. In 24 of the neonates, umbilical cord blood was also studied. Gel filtration was used to estimate the molecular size of IgA present in cord and neonatal serum. The arithmetic mean concentration of IgA (+/- S.E.M.) found in the 48 neonates was 2.6 +/- 1.45 mg/l. No significant difference was detected between the breast fed and artificially fed neonates. Only 7S IgA was detected in cord blood and in neonatal serum. Six days of exclusive breast feeding therefore has no influence on total serum IgA levels on the sixth day of neonatal life nor does it result in detectable circulating 11S IgA at that time.