Important proteins of blood plasma, importance in the organism (albumin, Ig, proteins acute phase, transport proteins).

Introduction

Plasma proteins play essential roles in maintaining homeostasis, immune defense, transport of substances, and response to injury or infection. These proteins are primarily synthesized in the liver (except immunoglobulins) and have diverse structures and functions. This document discusses the key types of plasma proteins: albumin, immunoglobulins, acute-phase proteins, and transport proteins.

1. Albumin

Albumin is the most abundant plasma protein, accounting for about 60% of total plasma proteins. It is synthesized by the liver.

Functions:

- Maintains oncotic (colloid osmotic) pressure, preventing edema.

- Serves as a transport protein for hormones (e.g., thyroxine), fatty acids, bilirubin, and drugs.

- Acts as a buffer, contributing to acid-base balance.

Clinical Relevance:

Hypoalbuminemia can result from liver disease, malnutrition, or nephrotic syndrome, leading to edema and impaired transport of substances.

2. Immunoglobulins (Ig)

immunoglobulin types

Immunoglobulins, or antibodies, are produced by plasma cells (differentiated B lymphocytes). They are key components of the adaptive immune system.

Major classes:

- IgG: Most abundant, provides long-term immunity.

- IgA: Found in mucosal areas and secretions.

- IgM: First antibody produced in an immune response.

- IgE: Involved in allergic reactions and defense against parasites.

- IgD: Role in B cell activation.

Functions:

- Neutralization of pathogens.

- Opsonization for phagocytosis.

- Activation of the complement system.

3. Acute-Phase Proteins

Acute-phase proteins are plasma proteins whose concentrations change significantly during inflammation or infection.

Positive acute-phase proteins (increase in concentration):

- C-reactive protein (CRP): Marker of inflammation, promotes complement activation.

- Fibrinogen: Promotes blood clotting.

- Serum amyloid A: Involved in immune cell recruitment.

Negative acute-phase proteins (decrease in concentration):

- Albumin, transferrin.

Clinical Relevance:

Measurement of CRP and other acute-phase proteins is useful in diagnosing and monitoring inflammatory diseases.

4. Transport Proteins

Transport proteins in plasma bind and carry various endogenous and exogenous substances.

Examples:

- Transferrin: Transports iron.

- Haptoglobin: Binds free hemoglobin.

- Ceruloplasmin: Transports copper and has oxidase activity.

- Lipoproteins (e.g., LDL, HDL): Transport lipids and cholesterol.

These proteins ensure safe and efficient distribution of essential molecules and prevent toxicity from free circulating substances.

Conclusion

Plasma proteins are crucial to many physiological processes including fluid balance, immunity, inflammation, and molecular transport. Their concentrations can serve as diagnostic markers and are essential for understanding systemic responses to disease and injury.

References

1. Guyton and Hall Textbook of Medical Physiology, 14th Edition

2. Lippincott Illustrated Reviews: Biochemistry, 7th Edition

3. Lehninger Principles of Biochemistry, 7th Edition