A radioimmunoassay of cellular surface antigens on living cells using iodinated soluble protein A from Staphylococcus aureus.

Soluble protein A from Staphylococcus aureus does carry great promise as a marker for cellular surface determinants, due to its specific reaction with, and high affinity for, most subclasses of mammalian IgG. In this article we present the different parameters involved in a radioimmunoassay using 125-I-labelled protein A. Using such an approach the actual technical procedures involved are reported in detail together with tests for mammalian alloantigens, including HL-A in the human, H-2 in the mouse and AgB antigenic sites in the rat, as this presents an unique opportunity to compare them with already widely used assays for transplantation antigens. The different parameters of the assay are analysed in view of measuring with precision quantities of cell-surface IgG molecules, thereby allowing possible determinations of antigenic site numbers in a new and simplified manner.