Note the three large patches of cellular infiltrates on the left side of this screen.(These are indicated by arrows. They are aggregates of leukocytes that stain blue. During the acute phase of pyelonephritis, the predominant leukocyte is the neutrophil. Note that the glomeruli are uninvolved. The inflammation is limited to the interstitium of the kidney.
Although not visible on this slide, bacteria are the presumed cause of this suppurrative inflammation. Also not visible at this magnification are the tubular necrosis and the neutrophilic casts.