Serologic Studies of Proteolytic Fragments of Rabbit Agglutinating Antibodies.

The reaction of the bivalent rabbit antibody molecule with papain (1, 2) or pepsin (3) and a reducing agent results in the release of two univalent fragments. The evidence for univalence is provided by the capacity of the fragments of hydrolyzed antiprotein antibody to inhibit specifically the homologous precipitin reaction (2), and by direct measurements of the maximal number of moles of hapten that can be bound by a mole of active "fragments" after hydrolysis of antihapten antibody with papain (4, 5, 6). The present communication describes another property of the univalent fragments, namely, their capacity to combine with, but not agglutinate, red blood cells of the appropriate antigenic type. Whether unfragmented 7S "incomplete" antibodies are univalent is not known. This appears unlikely, at least in the case of "incomplete" antiRho, since it is difficult to understand how treatment of red cells with trypsin or other proteolytic enzymes could render them agglutinable by univalent antibody. The data presented here indicate that univalent fragments of antibody can react specifically with antigens of erythrocytes and that the reaction does not cause agglutination, even when the cells are first treated with one of several proteolytic enzymes. That the fragments combine with the erythrocytes is shown by the occurrence of agglutination on subsequent addition of an untreated antibody specific for the univalent fragments.