Uptake and fate of exogenous immunoglobulin G in the perfused human placenta.

The steady-state transfer of 125I-labelled human or bovine immunoglobulin G from the maternal circulation and its fate during transport to the fetal circulation were studied over a 2 h period in an intact perfused human placental lobule. The 125I activity in the closed-circuit fetal circulation rose linearly throughout this time. The transfer rate, expressed as a percentage of the total activity administered, was 0.11% per hour for human IgG and 0.04% per hour for bovine IgG. 97-99% of the 125I activity in the maternal circulation was associated with high molecular weight protein which was characterised as IgG. The corresponding figure for the fetal side was 10-30%. It is concluded that human IgG is taken up in preference to bovine IgG at the maternal surface of the syncytiotrophoblast but subsequently the majority of the internalised immunoglobulin is broken down intracellularly and the fragments released into the fetal circulation.