Blood serum does not contain fibrin. It is the liquid portion of blood that remains after a clot has formed and been removed. During coagulation, clotting factors are consumed, and this absence of specific proteins is what defines serum.
The Role of Fibrin in Blood Clotting
Blood clotting begins when a blood vessel is injured, triggering a cascade of reactions. Circulating in the blood is a soluble protein produced by the liver called fibrinogen. In response to injury, an enzyme named thrombin is activated, which acts on the fibrinogen, converting it into insoluble fibrin strands.
These newly formed fibrin strands are fibrous proteins that assemble into a mesh-like network at the site of the wound. This fibrin mesh is the primary structural component of a blood clot. It traps platelets, red blood cells, and other components, creating a stable plug that stops bleeding and allows the vessel to heal.
The formation of this stable clot removes fibrinogen from the liquid part of the blood. The process is further stabilized by factor XIII, which cross-links the fibrin threads to strengthen the clot. As the wound heals, the clot is later dissolved.
Distinguishing Serum from Plasma
The main difference between serum and plasma is the presence of clotting factors. Plasma is the liquid component of blood prevented from clotting by an anticoagulant, constituting about 55% of total blood volume. Because coagulation is inhibited, plasma retains all its components, including soluble fibrinogen.
In contrast, serum is the fluid that remains after blood has clotted. Since the clotting process consumes fibrinogen to create the solid clot, the resulting serum lacks these proteins. This distinction is important for laboratory analysis.
Plasma is used for tests assessing the blood’s clotting ability, while serum is preferred for many other diagnostic tests. The volume of serum recovered from a blood sample is also less than that of plasma.
Composition and Laboratory Use of Serum
After the removal of clotting factors and blood cells, serum retains a mixture of other substances. Its composition includes water, electrolytes, antibodies, antigens, and hormones. It also contains proteins like albumin, which transports hormones and fatty acids, and globulins, which are part of the immune system.
The absence of fibrinogen and other clotting proteins makes serum useful for many diagnostic applications, as these proteins can interfere with test results. Serological tests, which detect antibodies and antigens, rely on serum to identify past or current infections.
Serum is also used in tests to measure levels of hormones, electrolytes, and biomarkers that can indicate disease, such as in metabolic studies or tissue-typing. Because it is more stable than plasma, it is often preferred for achieving reliable laboratory results.