serum

Serum is the clear, pale‑yellow fluid that remains after blood has clotted. It is the liquid portion of blood devoid of cells and without the clotting factors fibrinogen and other components found in plasma. Serum contains electrolytes, hormones, antibodies, enzymes, metabolites, and various proteins such as albumin and globulins.

By contrast, plasma is obtained from blood that has not clotted and still contains fibrinogen and clotting factors. Serum lacks these clotting proteins, which makes it suitable for many diagnostic tests. In addition to antibodies, serum carries substances released from tissues and pathogens, enabling a wide range of analyses.

In medicine and clinical laboratories, serum is widely used for diagnostic testing. It is commonly used in serology to detect antibodies, in chemistry panels to measure metabolites and enzymes, and for drug and hormone assays. Serum is obtained by allowing a blood sample to clot and then centrifuging to remove solids.

Therapeutically, sera and antisera are used to provide passive immunity against certain diseases or toxins. Historically, serum therapy played a major role in treating infections such as diphtheria, though modern applications are more limited and tightly regulated.

In biological research, fetal bovine serum (FBS) is a standard supplement added to cell culture media because of growth factors and nutrients. Its use raises ethical and variability concerns, and alternatives are actively explored.

In dermatology and consumer products, serums are concentrated topical preparations marketed to deliver active ingredients such as vitamin C, hyaluronic acid, or retinoids. These products are typically applied after cleansing and before moisturizers.