IgG receptors on foetal chick yolk sac

In the chicken, maternal IgG is deposited in the oocyte yolk and subsequently transported intact by the foetal chick yolk sac into the chick circulation. Such transport appears to require a specific membrane-localized IgG receptor. Using the assay of 125I-yolk IgG binding, we have investigated the existence and properties of such receptors in yolk sac tissue disks. We find that: (1) There are specific, 125I-IgG binding sites on yolk sac tissue; (2) specific yolk 125I-IgG binding has a distinct pH optimum at pH 6.0--6.3; (3) the apparent equilibrium dissociation constant of specific 125I-IgG binding is 2--3 X 10(-6) M; and (4) heterologous proteins compete less efficently than yolk IgG for 125I-IgG binding.