Permeability of the blood-air barrier to antiperoxidase antibodies and their fragments in the normal rat lung.

The permeability of the blood-air barrier to antiperoxidase (HRP) IgG antibodies (160,000 daltons), F(ab')2 fragments (100,000 daltons) and Fab fragments (50,000 daltons) was studied in the normal rat. The use of isologous and heterologous immunoglobulin G or IgG fragments injected intravenously allowed a sequential study. It was shown that these proteins transfer from the vascular bed towards to interstitium. Evidence was obtained that these proteins crossed the endothelium through interendothelial spaces or structures suggesting transendothelial channels. The alveolar epithelium was found to be an efficient barrier for heterologous and isologous proteins, raising the question of the origin of serum proteins found in the alveolar surface material. Heterologous anti-HRP IgG antibodies and their Fab fragments were also administered intraalveolarly. The alveolar epithelium was found permeable to Fab fragments but not to IgG molecules, suggesting that some serum proteins present in the alveolar lining fluid can be transepithelially reabsorbed.