A substance formed in the BLOOD as it clots. It is produced in threads, which form a meshwork and then contract to produce a dense, felted mass. This substance is formed not only from shed blood but also from LYMPH which exudes from the lymph vessels. Thus fibrin is found in all inflammatory conditions, and within serous cavities like the PLEURA, PERITONEUM, and PERICARDIUM, forming a thick coat upon the surface of the inflamed membranes. It is also found in inflamed joints, and in the lung as a result of pneumonia. (See COAGULATION.)