Plasma Protein – Definition, Function and Importance
Plasma proteins are proteins dissolved in blood plasma that perform vital functions, including transporting substances, regulating blood pressure, and supporting immune defence.
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Plasma proteins are proteins dissolved in blood plasma that perform vital functions, including transporting substances, regulating blood pressure, and supporting immune defence.
What Are Plasma Proteins?
Plasma proteins are proteins dissolved in the liquid component of blood known as blood plasma. They account for approximately 6–8% of plasma and comprise numerous protein groups with diverse biological functions. Most plasma proteins are synthesised in the liver, while certain proteins – particularly antibodies – are produced by immune cells.
Main Groups of Plasma Proteins
Albumins
Albumin is the most abundant plasma protein, making up approximately 55–60% of all plasma proteins. Its key functions include:
- Transport of fatty acids, hormones, bilirubin, and medications
- Maintenance of colloid osmotic pressure (preventing fluid from leaking out of blood vessels into surrounding tissues)
- Buffering capacity in acid-base balance
Globulins
Globulins form a heterogeneous group of plasma proteins subdivided into alpha, beta, and gamma globulins:
- Alpha globulins (e.g., alpha-1-antitrypsin, alpha-2-macroglobulin): protection against tissue degradation and transport functions
- Beta globulins (e.g., transferrin, complement proteins): transport of iron and lipids, activation of the immune system
- Gamma globulins (immunoglobulins / antibodies): key components of the specific immune defence, produced by B lymphocytes
Fibrinogen
Fibrinogen is a central protein in blood coagulation. When a blood vessel is injured, the enzyme thrombin converts fibrinogen into fibrin, which forms a clot and seals the wound.
Functions of Plasma Proteins at a Glance
- Transport: Binding and carrying hormones, vitamins, minerals, lipids, and drugs through the bloodstream
- Immune defence: Antibodies and complement proteins combat pathogens
- Blood coagulation: Clotting factors (e.g., fibrinogen, prothrombin) enable wound healing
- Maintenance of osmotic pressure: Prevents oedema (fluid accumulation in tissues)
- Buffering function: Stabilisation of blood pH levels
- Enzymatic functions: Some plasma proteins act as enzymes and catalyse biochemical reactions
Clinical Significance – Changes in Plasma Proteins
Hypoproteinaemia (Low Protein Levels)
A reduction in plasma proteins – particularly albumin – may result from:
- Malnutrition or undernutrition
- Liver disease (reduced protein synthesis)
- Kidney disease (protein loss through urine, e.g., in nephrotic syndrome)
- Chronic inflammation or malignant conditions
A common consequence is the development of oedema (fluid accumulation in tissues) due to a drop in colloid osmotic pressure.
Hyperproteinaemia (High Protein Levels)
Elevated plasma protein levels may occur in:
- Acute inflammatory responses (rise in acute-phase proteins such as CRP and fibrinogen)
- Certain cancers (e.g., multiple myeloma with increased monoclonal immunoglobulins)
- Dehydration (fluid deficiency)
Diagnosis and Laboratory Values
Plasma proteins are measured through blood tests. Important assessments include:
- Total protein: Normal range 60–80 g/L in blood serum
- Albumin: Normal range 35–55 g/L
- Protein electrophoresis: Separation and quantification of individual protein fractions (albumin, alpha-1, alpha-2, beta, and gamma globulins)
- Specific proteins: e.g., CRP, fibrinogen, immunoglobulins IgG, IgA, IgM
References
- Löffler, G. / Petrides, P. E. / Heinrich, P. C. (eds.): Biochemie und Pathobiochemie. 9th edition. Springer Medizin Verlag, 2014.
- Kasper, D. L. et al. (eds.): Harrison's Principles of Internal Medicine. 20th edition. McGraw-Hill Education, 2018.
- World Health Organization (WHO): Serum albumin levels as an indicator of nutritional status. WHO Technical Report, Geneva.
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