On "incomplete" Anti-rh Antibodies: Mechanism of Direct Agglutination Induced by Mercaptoethanol.

Since "incomplete" or "blocking" antibodies were first demonstrated in 1944 (1, 2), a variety of techniques has been developed to demonstrate the combination of incomplete antibodies with the appropriate erythrocytes (3). These procedures utilize high protein media, enzyme-treated cells, the antiglobulin (Coombs') reaction, hypotonic environments, or isotopic labeling. Previous explanations for the absence of direct agglutination of erythrocytes by "incomplete" antibodies dealt with the antigenic nature of the erythrocyte surface and the valency of the antibody molecule. Most investigators have concluded that the incomplete antibody molecule is bivalent (4-7). A more recent approach concerns the orientation of the combining sites of the incomplete antibody molecule. Pirofsky and Cordova (8, 9) postulated that the incomplete anti-Rh antibody molecule has one reactive group available for lattice formation and one or more potentially reactive groups that are not available because of steric inaccessibility. Their experiments showed that in the presence of mercaptoethanol incomplete anti-Rh serum produced direct saline agglutination of the appropriate cells. This finding suggested that the reduction of disulfide bonds alters the tertiary structure of the antibody molecule in such a way as to expose the blocked sites, which are then available for agglutination of erythrocytes in a saline medium.