During CAPD, the peritoneal cavity is submitted to situations such as the accidental bacterial contaminations and the continuous presence of dialysate, which stimulate immunological factors. The antibacterial defenses include opsonins and cells. Only IgG and fibronectin are present in the peritoneal effluent but they are diluted by the dialysate. Macrophages represent 70 to 80% of the peritoneal cells. Beside good phagocytic capacity, some may have a defective bactericidal activity. The continuous presence of dialysate leads to a chronic local inflammation. Macrophages and lymphocytes will synthesize IL-1, PGE2 and IFN-Y. These substances generate movement, attachment and proliferation of fibroblasts in the peritoneal submesothelial tissue, resulting in progressive fibrosis. This fibrosis, generally asymptomatic, may be responsible for loss of Ultrafiltration (UF) and Sclerosing Encapsulating Peritonitis (SEP).