Illustrated mechanism of development of cytological endometritis (CE). Panel A illustrates a normal situation; right after parturition (approximately 7 d postpartum), polymorphonuclear neutrophils (PMN) are mobilized into uterus to clean bacteria and placental debris, and later there are almost no PMN during the early lactation period (40 to 60 d postpartum). Panel B illustrates the situation where CE develops; if for some reason immune suppression occurs around parturition, less PMN are mobilized right after parturition and their activities are weak. Then the chronic inflammation continues, which results in the development of CE during early lactation. Adapted from Gilbert et al. (2007).