The specificity of a solid phase radioimmunoassay for human immunoglobulins

The specificity of a solid phase radioimmunoassay for immunoglobulin antigens, human Fab-mu in the case illustrated, has been assessed. The anti-Fab-mu serum available reacted well on gel diffusion with all Fab-mu- and light chain-containing proteins. In contrast the radioimmunoassay, in which unlabelled test antigen competes with radiolabelled Fab-mu for anti-Fab-mu coupled to Sephadex beads, was sensitive only to proteins containing entire Fab-mu regions. A double antibody radioimmunoassay showed comparable or greater sensitivity for Fab-mu-containing proteins, but was much less specific. By using Fab-gamma as the labeled antigen, with only light chain determinants reacting with the anti-Fab-mu, the solid phase assay was rendered polyspecific in being sensitive to all proteins containing light chain. Reasons for the specificity of the solid phase assay are discussed: steric factors can be expected in some cases to permit strong binding of antigen by multiple bonds, in others to restrict the number of molecular determinants simultaneously available to antibody.