A method for the detection and quantitation of Fc receptor sites on cells.

A new rosette technique for identification of Fc-receptor-bearing cells is based on the ability of sheep erythrocytes coated with protein A of Staphylococcus aureus (ES) to form rosettes with cells treated with monomeric IgG or aggregated IgG. The IgG is attached to lymphocytes through its Fc region and binds to a trypsin-resistant but pronase-sensitive receptor (considered an Fc receptor). The ES rosette technique facilitates studies of the interaction of IgG with Fc receptor sites; the binding of any IgG preparation reacting with protein A of Staphylococcus aureus (SpA) can be studied by the technique. Inhibition of rosette formation by SpA was used for quantitation of IgG fixed to the cell surface (that is, the number of Fc receptors/cell). The sensitivity of the method permits quantitation of less than 10(5) IgG molecules bound to the Fc receptors. The relative affinity constant between Fc receptors and IgG ligands was estimated by plotting the percentage of ES rosettes as a function of IgG concentration and calculating the reciprocal of the IgG concentration giving half the maximal number of ES rosettes.