The immune system plays a significant role in the pathogenesis of retrograde menstruation and endometriosis. In a typical immune response, macrophages and other immune cells should clear away the menstrual debris that enters the pelvic cavity. However, in some women, this process is inefficient, allowing endometrial cells to implant and proliferate. Histological studies often reveal increased numbers of activated macrophages and other immune cells in the peritoneal fluid of women with endometriosis, suggesting an altered immune environment.
