Sentence:  Microscopically , both kidneys showed many small cortical scars in which there was glomerular and interstitial fibrosis , tubular atrophy , and an <WSD>infiltration</WSD> of lymphocytes and plasma cells . 
Meaning of the word marked with <WSD> tags:  a process in which individuals (or small groups) penetrate an area (especially the military penetration of enemy positions without detection)