Chapter Notes: Plasma Proteins and Immunoglobulins
This chapter explains the composition, synthesis, separation, physiological roles and clinical significance of major plasma proteins, and describes the structure, classes, functions and disorders of immunoglobulins. The text is organised to help students understand normal values, laboratory separation patterns, and how protein abnormalities relate to disease.
Plasma Proteins
Overview
Plasma contains a large number of proteins with diverse functions. More than 100 plasma proteins have been described; this chapter emphasises those present in relatively high concentration or of major clinical importance: albumin, globulins and fibrinogen. Plasma proteins are involved in maintenance of colloid (oncotic) pressure, transport of substances, defence mechanisms, coagulation and enzymatic regulation.
Normal concentrations
| Analyte | Typical range |
|---|---|
| Total protein (serum) | 6.0-8.0 g/dL |
| Serum albumin | 3.5-6.0 g/dL |
| Serum globulin | 2.0-3.5 g/dL |
| Fibrinogen | 200-400 mg/dL |
| A/G ratio (albumin : globulin) | 1.2 : 1 to 2.5 : 1 |
Synthesis of plasma proteins
Most plasma proteins are synthesised in the liver. Albumin and fibrinogen are essentially liver products. Approximately 50-80% of globulins are synthesised in the liver; the remaining globulins, particularly the γ-globulins (immunoglobulins), are produced by lymphoid tissues, plasma cells and some lymphocytes.
Because albumin is produced exclusively in the liver, serum albumin concentration is a useful indicator of hepatic synthetic function. In severe liver disease albumin decreases markedly, whereas γ-globulin levels may remain unchanged or even increase (since these are synthesised outside the liver), altering the A/G ratio.
Separation of plasma proteins
Common laboratory methods used to separate and analyse plasma or serum proteins include:
- Salting out (fractional precipitation)
- Electrophoresis (most commonly used clinically)
- Ultracentrifugation
- Immunoelectrophoresis and related immunoassays
Serum protein electrophoresis
When serum proteins are separated by electrophoresis (for example on cellulose acetate or agarose), five principal bands or fractions are observed in normal serum. From fastest to slowest migration they are:
- Albumin
- α1-globulin
- α2-globulin
- β-globulin
- γ-globulin
Each band produces a peak on densitometric scanning. Characteristic changes in the relative heights and shapes of these peaks provide diagnostic information.
Typical electrophoretic patterns in disease
- Infective hepatitis: slight decrease in albumin and significant increase in γ-globulins.
- Liver cirrhosis: decrease in albumin with elevations in β and γ-globulins (polyclonal increase).
- Nephrotic syndrome: marked hypoalbuminaemia, increase in α2-globulin and raised β-globulin.
- Multiple myeloma: a sharp monoclonal spike in the γ region (monoclonal gammopathy).
Major classes of plasma proteins
Albumin
Albumin is a globular protein of approximately 69 kDa (about 585 amino acid residues). It is the most abundant plasma protein, accounting for roughly 50% of total plasma protein mass. Albumin is synthesised exclusively by the liver and has a plasma half-life of about 15-20 days.
Functions of albumin
- Oncotic (colloid) pressure: Albumin makes the largest contribution to plasma oncotic pressure and so plays a principal role in maintaining plasma volume and distribution of body fluids. Severe hypoalbuminaemia leads to oedema.
- Transport: Albumin binds and transports numerous poorly water-soluble molecules including free fatty acids, unconjugated bilirubin, many drugs and some steroid hormones. It also carries some cations such as calcium (bound fraction).
- Buffering: Albumin contributes to the protein buffering capacity of plasma because of its high concentration and amino acid side chains capable of accepting or donating protons.
- Nutritional reserve: During severe malnutrition albumin can be catabolised to supply amino acids.
Clinical significance of albumin
- Hypoalbuminaemia is seen in malnutrition, nephrotic syndrome (protein loss in urine) and advanced liver disease (reduced synthesis).
- Hyperalbuminaemia typically reflects haemoconcentration (acute dehydration) and has little direct pathological significance.
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Globulins
Globulins are a heterogeneous group of proteins that migrate in α1, α2, β and γ regions on electrophoresis. They include protease inhibitors, transport proteins, acute phase reactants, immunoglobulins and other defence molecules.
| Class / fraction | Examples | Principal functions |
|---|---|---|
| α1-globulin | α1-antitrypsin (α1-protease inhibitor) | Inhibits proteases (e.g., elastase); protects lung elastin from proteolysis |
| α2-globulin | α2-macroglobulin, haptoglobin, ceruloplasmin, hemopexin | Broad protease inhibition (α2-macroglobulin), binds free haemoglobin (haptoglobin), copper transport and ferroxidase activity (ceruloplasmin), binds free heme (hemopexin) |
| β-globulin | Transferrin, complement components, some acute phase proteins | Iron transport (transferrin), immune and inflammatory functions |
| γ-globulin | Immunoglobulins (IgG, IgA, IgM, IgD, IgE) | Humoral immune defence (antibodies) |
Selected globulin proteins and clinical notes
- α1-antitrypsin (AAT): a major antiprotease that neutralises elastase. Genetic deficiency of AAT predisposes to pulmonary emphysema; cigarette smoke also impairs AAT function and increases emphysema risk.
- Ceruloplasmin: a copper-containing ferroxidase that transports most plasma copper and participates in iron metabolism. Levels are reduced in Wilson's disease and in some malnutrition or nephrotic syndrome cases.
- Haptoglobin: binds free haemoglobin released from haemolysis to form a complex too large to be filtered by the kidney, conserving iron and preventing renal damage.
- α2-macroglobulin: a broad-spectrum protease inhibitor (inhibits trypsin, chymotrypsin, thrombin and others). Levels increase in nephrotic syndrome and certain inflammatory states; they fall in some myelomas.
- Hemopexin: binds free heme (not haemoglobin) and prevents iron loss in urine by forming a high-molecular-weight complex.
- Transferrin: iron transport protein; each transferrin binds two Fe3+ ions. Transferrin is increased in iron deficiency and pregnancy, and decreased in chronic infection and malnutrition.
- C-reactive protein (CRP): an acute phase reactant that reacts with C-polysaccharide of pneumococci; CRP rises markedly in bacterial infection and acute inflammation and can help distinguish bacterial from viral infections.
- β2-microglobulin: a component of the major histocompatibility complex (MHC) class I system. Plasma levels rise with malignant lymphoid/myeloid proliferation and in renal failure.
Fibrinogen
Fibrinogen (clotting factor I) is a liver-synthesised glycoprotein comprising about 4% of plasma protein. During coagulation fibrinogen is enzymatically converted by thrombin to fibrin monomers, which polymerise to form the insoluble fibrin clot. Plasma fibrinogen decreases in severe hepatic disease due to reduced synthesis.
Immunoglobulins (Ig)
General features
Immunoglobulins (γ-globulins) are glycoproteins that function as antibodies. All antibodies are immunoglobulins, but not every immunoglobulin molecule is necessarily an antibody with a specific neutralising function. Immunoglobulins are produced primarily by plasma cells and to some extent by B lymphocytes.
Basic structure
An immunoglobulin molecule is typically a Y-shaped structure composed of four polypeptide chains: two identical heavy (H) chains and two identical light (L) chains linked by disulfide bonds. Light chains have a molecular weight ≈ 25 kDa; heavy chains range ≈ 50-75 kDa depending on the class. Each L and H chain has variable (V) and constant (C) regions. The variable regions of one heavy and one light chain form an antigen-binding site. Two identical antigen-binding sites are present on most monomeric immunoglobulins.
Enzymatic cleavage with papain splits Ig into two Fab fragments (antigen binding) and one Fc fragment (crystallisable fragment that interacts with complement and Fc receptors). The hinge region between CH1 and CH2 of heavy chains gives flexibility for antigen binding.
Light chains are of two types: κ (kappa) or λ (lambda); a given immunoglobulin molecule contains either κ or λ light chains, not both.
Immunoglobulin classes
| Class | Heavy chain |
|---|---|
| IgG | γ (gamma) |
| IgA | α (alpha) |
| IgM | μ (mu) |
| IgD | δ (delta) |
| IgE | ε (epsilon) |
Classes - structure, abundance and principal functions
| Type | Structure | Approx. percentage of serum Ig | Key characteristics and functions |
|---|---|---|---|
| IgG | Monomer | ≈ 70% | Major serum antibody. Crosses the placenta (maternal IgG confers passive immunity to fetus). Important in secondary immune responses; neutralises toxins and viruses, opsonises microbes, activates complement (via Fc) and promotes phagocytosis. |
| IgA | Monomer (serum) or dimer (secretory IgA with J chain) | ≈ 20% | Major immunoglobulin in external secretions (saliva, tears, colostrum, respiratory and intestinal secretions). Prevents attachment of pathogens to mucosal surfaces and provides mucosal immunity. |
| IgM | Pentamer (joined by J chain); monomer on B-cell surface | ≈ 8-10% | First antibody produced in primary immune response. Effective at agglutination and complement activation. Serves as B-cell receptor in membrane form. |
| IgD | Monomer | < 1% | Present on B-cell surfaces; may function as an antigen receptor. Circulating levels are very low and specific effector functions are limited. |
| IgE | Monomer | Trace (≈ 0.004%) | Responsible for immediate (type I) hypersensitivity and allergic reactions by binding to Fc receptors on mast cells and basophils and triggering histamine release. Also involved in defence against helminthic parasites. |
Molecular forms and special points
- IgM is pentameric in serum (five monomer units + J chain), giving it ten antigen binding sites; as a pentamer it is especially effective at agglutinating antigens and activating complement.
- IgA dimeric secretory form has a secretory component that protects it from proteolysis in mucosal secretions.
- Fab and Fc regions: Fab binds antigen; Fc mediates downstream effector functions such as complement activation and interaction with Fc receptors on phagocytes or natural killer cells.
Functions of antibodies
Antibodies protect the host by several mechanisms:
- Neutralisation of toxins and viruses by blocking binding sites.
- Opsonisation - coating of microbes to enhance phagocytosis by macrophages and neutrophils.
- Activation of the classical complement pathway leading to opsonisation, inflammation and lysis of some pathogens.
- Agglutination and precipitation of soluble antigens to facilitate clearance.
- Antibody-dependent cell-mediated cytotoxicity (ADCC) - NK cells recognise antibody-coated targets and release cytotoxic granules.
- Some antibodies have catalytic or microbicidal activities (rare and specialised functions).
Monoclonal and polyclonal antibodies
Polyclonal antibodies are heterogeneous antibodies produced by many different B-cell clones in response to an antigen; they bind multiple epitopes on the same antigen. Monoclonal antibodies are homogeneous antibodies produced by identical B cells (or hybridoma cell lines) directed against a single epitope; monoclonal antibodies are widely used diagnostically and therapeutically.
Disorders related to immunoglobulins and plasma proteins
Multiple myeloma
Multiple myeloma is a clonal neoplasm of plasma cells resulting in high concentrations of a single (monoclonal) immunoglobulin or immunoglobulin fragment. The most common are monoclonal IgG or IgA. On serum protein electrophoresis this produces a sharp, narrow spike (M spike) in the γ region. Clinical consequences include bone lesions, hypercalcaemia, renal impairment and susceptibility to infection.
Bence Jones proteins and light chain disease
In some multiple myeloma cases plasma cells produce excess immunoglobulin light chains (κ or λ) that are small enough to be filtered by the glomerulus and appear in urine as Bence Jones proteins. These light chains characteristically precipitate when heated in a urine sample over a certain temperature range and redissolve on further heating. Myeloma composed primarily of light chains is sometimes called light chain disease.
Amyloidosis
In some patients with plasma cell dyscrasias fragments of immunoglobulin light chains deposit in tissues as amyloid fibrils, causing organ dysfunction (amyloidosis).
Immunoglobulin deficiency states
Congenital or acquired deficiencies of immunoglobulins (for example agammaglobulinaemia, common variable immunodeficiency) lead to increased susceptibility to infections. Replacement therapy with immunoglobulin (intravenous immunoglobulin) may be used in some conditions.
Key clinical correlations and laboratory notes
- Serum albumin measurement is an important part of liver function assessment and nutritional evaluation.
- Electrophoresis patterns support diagnosis of inflammatory states, chronic liver disease, nephrotic syndrome and monoclonal gammopathies.
- Acute phase proteins (for example CRP) rise rapidly in bacterial infection and inflammation and are useful markers of acute disease activity.
- Measurement of specific immunoglobulin classes and monoclonal protein detection (serum/urine protein electrophoresis and immunofixation) are essential in evaluation of multiple myeloma and related disorders.
Summary: Plasma proteins - especially albumin, a variety of globulins and fibrinogen - perform essential roles in fluid balance, transport, defence and coagulation. Immunoglobulins are specialised γ-globulins that mediate humoral immunity; their structure, class distribution and levels are critical diagnostic and functional markers in health and disease.
