1. To investigate the relative degree to which human IgG and other plasma proteins bind to cell membranes of the full-term human placenta, suspensions of membranes mixed with radio-iodinated proteins were incubated at pH 6·5 and subjected to Sepharose 2B column chromatography. The amount of labelled protein associated with membrane protein in membrane-containing fractions of the eluate was then determined.

2. Binding of IgG and each of the four subclasses of IgG was appreciable. Binding of IgG was markedly reduced if membranes were incubated at pH 8·0 rather than 6·5. Binding of labelled IgG was inhibited by excess of unlabelled IgG but not by excess of unlabelled albumin. Placental membranes bound much more IgG than did erythrocyte membranes.

3. Binding of insulin was relatively greater than that of IgG, whereas binding of IgE, IgM, IgA, albumin, transferrin and polyvinylpyrrolidone was much less than that of IgG and each of its subclasses.

4. The relative binding of IgG and fragments of the IgG molecule was, in decreasing order of magnitude: light chains, IgG, Fc, heavy chains and F(ab′)2.

5. The results are consistent with Brambell's hypothesis for the mechanism of the transmission of passive immunity from mother to young. In particular the data are consistent with the existence of a limited number of IgG specific receptors on the microvillus membrane of the syncytial trophoblast. Such receptors could protect IgG molecules from degradation during their transplacental transport. The submolecular region of the IgG molecule involved in specific membrane binding appears to be common to all four subclasses and to include the Fc region.

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