C3 and erythema multiforme: In addition to estradiol, a potential factor for the upregulation of C3 expression by endometrial cells was the peritoneal fluid rich in pro-inflammatory factors (113); TNF-α and IL-1β levels appeared to be increased in the peritoneal fluid of patients with EM, and higher levels of TNF-α seemed to be associated with more advanced stages of the disease (113–116).