Plasma is the blood fluid that carries blood clotting agents, while serum is the water fluid from blood without the clotting factors. Fibrinogen is a clotting agent in plasma.
Plasma constitutes about 50 percent of blood content and contains proteins that help in the transportation of material such as glucose and other dissolved nutrients through the blood. It surrounds white and red blood cells and platelets throughout the body. Plasma aids in the maintenance of blood pressure and in the regulation of body temperature. Serum contains proteins, electrolytes, antibodies, antigens and hormones. The fluid is studied in serology and is used in diagnostic tests and blood typing.