REACTION OF I-131 TRACE LABELED HUMAN ANTI-RH0(D) WITH RED CELLS*†

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Abstract
Trace labeled I131 anti-Rho (D) was prepared by iodinating isoantibody-containing fractions obtained by alcohol fractionation of high titered anti-Rh0 (D) serum. An I131 anti-RhQ (D) was prepared by eluting the anti-Rh0 (D) from red cell stroma sensitized with the I131 alcohol fractions (II + III). Evidence for specificity of the I131 bound to Rho (D) positive cells consists of the following observations. Rh0 (D) negative red cells bind less than 3% for the I131 taken up by Rh0 (D) positive red cells. There is selective removal of the I131 bound to Rh0 (D) positive red cells by Rh0 (D) positive red cells. A direct proportionality exists between the amount of I131 bound to the Rh0 (D) positive red cells and the concentration of red cells. Complete inhibition of the binding of I131 occurs when the red cells have been pretreated (sensitized) with unlabeled anti-Rh0 (D) serum. The reaction between I131 anti-Rh0 (D) and Rh0 (D) positive red cells has been characterized as a first order reaction with a rate constant of 5.78% per minute. The temperature and pH optima as well as the thermal stability of the I131 anti-Rh0 (D) preparations are similar to those found with the native anti-Rh0 (D). The I131 bound to red cells has been quantitatively recovered in the stroma after hemolysis of I131 sensitized red cells. An estimate made from these data indicates that there are 2,000 to 3,000 Rh0 (D) antigen sites on each red cell.
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