High Quality Content by WIKIPEDIA articles! Apical periodontitis is typically the body’s defense response to the threat of microbial invasion from the root canal. Primary among the members of the host defense mechanism is the polymorphonuclear leukocyte, otherwise known as the neutrophil. The task of the neutrophil is to locate and destroy microbes that intrude into the body – anywhere in the body – and they represent the hallmark of acute inflammation.