The renal lesions of serum sickness were studied with the electron microscope. The most prominent change was a marked swelling and proliferation of glomernlar endothelial cells causing obliteration of the capillary lumen. The basement membrane also showed focal thickenings and excrescences. Deposits of electron-dense material blended into the basement membrane. On the extracapillary side epithelial foot processes were reduced in number and replaced by broad sheets of cytoplasm which were closely applied to the basement membrane. From a comparison of electron and fluorescent microscopic studies of the glomerulus in serum sickness, it would seem that antigen-antibody complexes initiated injury in endothelial cells, although the possibility of the primary reaction occurring on basement membrane cannot be excluded.

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