A direct correlation between the amount of kidney-fixing antibody and the degree of associated renal injury was demonstrated for rabbit and duck nephrotoxic antibodies. No evidence for a qualitative difference among nephrotoxic antibodies of a given type was obtained. It appeared that duck nephrotoxic antibody was directed against a wider spectrum of rat renal antigens than was rabbit nephrotoxic antibody. In order to produce immediate proteinuria an amount of rabbit or duck gamma-2 kidney-fixing antibody capable of occupying approximately 45 per cent or more of the capillary filtration surface was needed. For immediate proteinuria an amount of rabbit gamma-2 kidney-fixing antibody capable of reacting with more than one-half of the available antigenic sites was needed. Less than twice that amount of rabbit antibody completely saturated available antigenic sites in the kidney. Virtually all nephrotoxic antibodies in hyperimmune rabbit nephrotoxic sera were of the gamma-2 variety while nephrotoxic antibodies in comparable duck nephrotoxic sera were found in gamma-2 (with 5.8 and 7.4S sedimentation coefficients) and gamma-1M fractions.

Gamma-1M duck nephrotoxic antibody was 60 times more potent a nephritogen than gamma-2 duck nephrotoxic antibody on a molecular basis. Mercaptoethanol abolished the nephrotoxicity of gamma-1M duck antibody and reduced that of gamma-2 duck antibodies but had no effect on rabbit gamma-2. In no case did mercaptoethanol treatment have an effect on the kidney-fixing properties of the antibodies involved.

After injection of nephrotoxic antibodies there appeared to be a prompt fixation of a majority of the antibody to tissue antigens primarily in the kidney. However, a small amount of potentially kidney-fixing antibody remained in the circulation for a considerable period apparently reflecting a dissociation of less avid antibodies with an equilibrium between fixed and free antibody. The role of this more easily dissociable antibody in the progression of nephrotoxic nephritis is not certain but it is possible that it could play a role in the early progression of the disease.

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