A light yellow, transparent liquid formed when blood coagulates. When blood is drawn from a vessel and placed in a test tube without an anticoagulant, the coagulation process begins immediately. The blood quickly solidifies into a gel-like clot, which then contracts. Around this clot, a pale yellow, transparent fluid called serum is left behind. This serum can also be obtained by centrifuging the coagulated blood. During coagulation, fibrinogen is converted into fibrin, forming a stable clot, which means that serum does not contain fibrinogen—this is one of the main differences between serum and plasma. Additionally, platelets release various substances during the coagulation process, and many clotting factors undergo changes. These components remain in the serum and continue to alter over time, such as prothrombin being converted into thrombin, which gradually decreases or disappears with prolonged storage. These dynamic changes distinguish serum from plasma. However, many other substances that did not participate in the coagulation process are similar to those found in plasma. To avoid interference from anticoagulants, serum is often used as a sample for analyzing various chemical components in blood.

Main Functions:

● Providing essential nutrients: Serum contains amino acids, vitamins, inorganic salts, lipids, and nucleotide derivatives, all of which are crucial for cell growth and survival. ● Supplying hormones and growth factors: It includes insulin, adrenocortical hormones like dexamethasone, steroid hormones such as estradiol, testosterone, and progesterone, as well as growth factors like fibroblast growth factor, epidermal growth factor, and platelet-derived growth factor. ● Delivering binding proteins: Proteins like albumin transport important low-molecular-weight substances, including vitamins, fats, and hormones. Transferrin, for example, carries iron. These binding proteins play a key role in cellular metabolism and function. ● Offering protective factors: Serum contains factors that help prevent mechanical damage to cells, especially during handling or culture. ● Protecting cultured cells: Certain cell types, such as endothelial and bone marrow-like cells, release proteases. Serum contains anti-protease components that neutralize these enzymes. This property was discovered accidentally but is now intentionally used to stop trypsin digestion during cell passage. Trypsin is widely used to detach adherent cells, and serum helps protect them from mechanical stress, particularly in suspension cultures. Serum also contains trace elements and ions, such as selenium (SeO3), which are vital for metabolic processes and detoxification.

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