Acute interstitial nephritis is found in the infectious diseases of children, particularly in diphtheria and scarlet fever, but may be met with in other infectious diseases.

The disease is characterized by general and focal infiltration of the interstitial tissue of the kidney with cells which correspond to those which Unna has described under the name of plasma cells. The focal character of the infiltration is marked; even in the cases in which all parts of the kidney show some interstitial cellular infiltration the cells are most abundant in certain foci. These foci are found in three places: in the boundary zone of the pyramids, in the sub-capsular region of the cortex, and around the glomeruli. A considerable number of cases is found in which the blood-vessels of the boundary zone of the pyramids contain nnmbers of lymphoid and plasma cells without any infiltration of the interstitial tissue. The new cells in the interstitial tissue are due to emigration from the blood-vessels and multiplication by mitotic division of the cells which have emigrated. The cells can emigrate as plasma. cells or as lymphoid cells, and the latter may change into plasma cells in the tissues. In the normal individual, plasma cells may be formed in the mucous membrane of the intestine, where they practically form the entire tissue between the epithelium and the muscularis mucosa, and to a limited extent in the spleen. In diphtheria, in scarlet fever, and probably in a number of infectious diseases, plasma cells are formed in great numbers in the spleen and bone-marrow, and to some extent in the lymphatic glands. In the spleen they are formed from the cells of the Malpighian bodies, which are often principally composed of them, and to some extent from the cells in the pulp. They are formed from the lymphoid cells.

No adequate explanation is found for the focal character of the lesions in the kidneys. There is some ground for believing that the physical conditions of the circulation may have something to do with their accumulation in the vessels in certain places. It is also possible that in the interstitial foci there may be soluble substances which exert a positive chemotaxis for them. Such substances may be foundin the urine, which may exert its influence on the interstitial tissue in different places. The explanation of the foci cannot be found in primary focal degeneration of the epithelium. Epithclial degeneration in these cases is always present, but it is diffuse. In foci where it is more intense and due to the interstitial changes, polynuclear leucocytes are found in the tissue, in the degenerated epithellum and in the tubules. Folynuclear lencocytes and not plasma cells are attracted by degenerated tissue. The foci are not due in these cases to the local action of bacteria.

In a number of the cases in which interstitial nephritis was found the kidneys were shown to be sterile both by cultures and by microscopical examination. In cases where bacteria were present they were found only in small numbers in cultures and not on microscopical examination, and their connection with the foci could not be demonstrated.

In three cases plasma cells were found in the interstitial tissue in definite bacterial diseases of the kidney. In these cases they were not found in connection with the lesions produced directly by the bacteria, but in the periphery of the purulent foci.