Globulins can be divided into three fractions based on their electrophoretic mobility using electrophoresis. Most of the α and β globulins are synthesized by the liver, whereas γ globulins are produced by lymphocytes and plasma cells in lymphoid tissue. α globulins consist of α-1 and α-2 globulins, and β globulins consist of β-1 and β-2 globulins. A few examples of globulin proteins are found in the table below. The third fraction known as γ globulins consists of the immunoglobulins: IgM, IgA, and IgG.
| Protein | kDa | Functions (partial list) |
Response in disease |
|
| α-1 | α-1 antitrypsin | 45 | Inhibits trypsin, anti-inflammatory | ↑ acute infl. disease ↓ hereditary deficiency (humans) |
| α-1 antichymotrypsin | 68 | Inhibits chymotrypsin, anti-inflammatory | ↑ acute infl. disease | |
| α-1 acid glycoprotein | 44 | anti-inflammatory andimmunomodulatory functions | ↑ acute infl. disease | |
| α-1 lipoprotein (HDL) | 180-350 | Lipid transport – reverse cholesterol transport | ↑ causes of hypercholesterolemia (inherited, nephrotic syndrome, DM, pancreatitis, hypothyroidism, cholestasis) ↓ liver disease |
|
| α-2 | Antithrombin (AT) | 65 | Inhibitor of thrombin and other clotting factors | ↑ acute phase response (possible in cats) ↓ in DIC, liver disease, protein-losing disorders (renal, GI) |
| α-2 macroglobulin | 820 | Protease inhibitor, anti-inflammatory | ↑ nephrotic syndrome, chronic active liver disease, acute infl. disease | |
| Haptoglobin | 80-160 | Hb binding, antibacterial effect | ↑ acute infl. disease, glucocorticoids in dogs ↓ hemolytic anemia |
|
| Ceruloplasmin | 151 | Copper transport, ferroxidase | ↑ acute infl. disease | |
| Protein C | 62 | inhibitor of activated coagulation factors FVIII and FV | ↓ sepsis, portosystemic shunts, liver disease, inherited (human and horse) | |
| α-2 lipoprotein (VLDL) May migrate as early b-1 in some species. | 1000 | Lipid transport – triglyceride transport | ↑ causes of hypertriglyceridemia and hypercholesterolemia (inherited lipoprotein disorders, nephrotic syndrome, DM, hypothyroidism, HAC, pancreatitis, hepatic lipidosis) | |
| β-1 | Transferrin | 76 | Iron transport | ± iron def., acute liver necrosis ↓ infl. disease, chronic liver disease, iron overload |
| Hemopexin | 80 | Heme scavenger | ↓ hemolytic disease, chronic active liver disease | |
| IgG4 | Anti-inflammatory | Can migrate in this region when expanded as a restricted gammopathy (e.g. heartworm disease) | ||
| β-2 | Fibrinogen | 340 | Fibrin precursor | ↑ acute infl. disease ↓ DIC, severe liver disease |
| Complement factor 3a | 180 | Pro-inflammatory, chemotactic substance | ↑ acute infl. disease | |
| β-2 lipoprotein (LDL) | 2400 | Lipid transport – cholesterol transport | ↑ causes of hypercholesterolemia (inherited, nephrotic syndrome, DM, pancreatitis, hypothyroidism, HAC, cholestasis) | |
| C-reactive protein | 140 | On bacteria, promotes the binding of complement | ↑ acute infl. disease | |
| IgM | 900 | Antigen specific binding | ↑ infectious or inflammatory dis., liver disease, B cell neoplasia (lymphoma, macroglobulinemia) ↓ deficiency syndromes |
|
| IgA | 160 | Antigen specific binding | ↑ infectious or inflammatory dis. (particularly mucosa), liver disease, plasma cell neoplasia (EMP, MM) ↓ newborns, deficiency syndromes |
|
| IgG4 | Anti-inflammatory | This subclass of IgG can migrate in this region with inflammatory conditions (e.g. heartworm infection, IgG4-related disease) | ||
| γ | IgG | 150 | Antigen specific binding | ↑ infectious or inflammatory dis., liver disease, B or plasma cell neoplasia (EMP, MM, lymphoma) ↓ newborns, deficiency syndromes |
| IgM and IgA also migrate here | As described above | |||
Physiology
The globulins consist of all non-albumin proteins. In contrast to the situation with albumin, which is a single protein, there are hundreds of different proteins included in the globulins. Although knowledge of the total globulin concentration is useful, it gives no information about the distribution of the different types of proteins within that total. Included in the globulins are specific groups of proteins that are produced in response to inflammatory stimuli. These include the acute phase proteins and the immunoglobulins (Igs). As such, important information aiding in the diagnosis and monitoring of inflammatory conditions can be obtained by further characterizing the globulin fraction by electrophoresis (ELP) of serum proteins (SPE). While only a few electrophoretic patterns are pathognomic for a specific disease, useful information of a more general nature often can be found.
Methodology
The globulin value on the chemistry panel is not measured, but is calculated by the equation:
Globulins = Total protein – Albumin
The principal behind this method of quantification is the fact that all serum proteins, except albumin, are considered to be globulins. The certainty of the globulin concentration is limited by the accuracy of calculated total protein and albumin concentrations. To circumvent this issue globulins can also be measured quantitively and qualitatively with electrophoresis. Radial immunodiffusion is used for accurate quantification of immunoglobulins and has also replaced immunoelectophoresis for determining the immunoglobulin comprising a monoclonal gammopathy.
Units of measurement (link to conversion calculator)
The concentration of globulins is measured in g/dL (conventional units) and g/L (SI units). The conversion equation is shown below:
mg/dL x 10 = g/L
Sample considerations
Sample type
Serum and plasma. Note that plasma globulin concentrations are higher than serum because plasma contains fibrinogen (a beta globulin), which is consumed during clotting to produce serum
Anticoagulant
EDTA or heparin can be used to measure globulins.
Stability
Globulins are considered to be relatively stable if samples are kept refrigerated.
Interferences
- Lipemia, hemolysis, and icterus: The interferences of measurement depends on that of total protein and albumin.
Test interpretation
Increased globulin concentration (hyperglobulinemia)
When we see an increase in total globulins, we usually think of antigenic stimulation (increasing immunoglobulins) or inflammation (increasing acute phase proteins, fibrinogen if globulin concentrations were measured in plasma, or immunoglobulins in an acquired immune response). Of course, plasma cell or B cell neoplasia can result in an expansion of a monoclonal immunoglobulin, which often results in a concurrent hyperglobulinemia. If there are concurrent increases in albumin concentration, we also consider fluid losses, resulting in dehydration and contraction of plasma water volume. Further interpretation of increased globulin concentrations requires electrophoresis to determine which globulin fractions are increased. Electrophoresis is not usually performed unless there is a sustained increase in globulins with no obvious cause (inflammation or other causes of antigenic stimulation) or if there is suspicion for underlying plasma or B cell neoplasia causing a paraneoplastic increase in immunoglobulins.
Increases in total globulins can result from increases in any or all of the fractions as determined by electrophoresis.
- Artifact: Dehydration of the sample (e.g. poorly capped tube with loss of fluid from plasma or serum) can result in falsely increased globulin concentrations.
- Pathophysologic
- α-Globulins
- Acute phase reactant response: This usually results in increased α (especially α-2) globulins. Acute phase reactants are a diverse group of proteins that increase in serum very rapidly (within 12-24 hours) following tissue injury of any cause (inflammation, acute bacterial and viral infections, necrosis, neoplasia, trauma). Raised serum levels are the result of increased hepatic synthesis mediated by cytokines (IL-1, IL-6, TNFα ). They also tend to remain elevated in chronic inflammatory conditions.
- Nephrotic syndrome: A dramatic increase in α-2 globulins is often seen (due to VLDL and α-2 macroglobulin).
- β-Globulins
- Inflammation (acute and chronic): increased β globulins often accompanies increases in γ globulins (response to antigenic stimulation).
- Active liver disease and suppurative dermatopathies: Both of which are associated with elevated IgM.
- Nephrotic syndrome: Associated with an increase in transferrin and lipoproteins.
- γ-Globulins
- Increases in this fraction occur most commonly in conditions in which there is an active immune response to antigenic stimulation usually resulting in a polyclonal gammopathy. Neoplasms of immunoglobulin-producing cells (plasma cells, B-lymphocytes) can also be responsible for monoclonal increases in this fraction. For more information, see total protein electrophoresis.
- Polyclonal gammopathy: This is seen as a broad-based peak in the β and/or γ region (arrow on image to the right). Some common causes include various chronic inflammatory diseases (infectious, immune-mediated), liver disease, FIPV (α-2 globulins are often concurrently elevated), occult heartworm disease, and Ehrlichiosis.
- Monoclonal gammopathy: This is seen as a sharp spike in the β or γ region. The peak can be compared to the albumin peak – a monoclonal gammopathy has a peak as narrow as that of albumin (see image below to the right). The usual cause of monoclonal gammopathies is neoplasia of B cells or plasma cells, although cases of non-neoplastic monoclonal gammopathy have been reported (see below for more information).
- Neoplasia: Multiple myeloma is the most common cause (producing an IgG or IgA monoclonal). Other neoplastic disorders associated with a monoclonal gammopathy include lymphoma (IgM or IgG) and chronic lymphocytic leukemia (usually IgG). Extramedullary plasmacytomas are solid tumors composed of plasma cells that are usually found in the skin of dogs. They have also been reported in the gastrointestinal tract and liver of cats and dogs. They can be associated with a monoclonal gammopathy, or even a biclonal gammopathy (if there are multiple tumors).
An increase in IgM is called macroglobulinemia. Waldenström’s macroglobulinemia is a neoplasm of B-cells (lymphoma) that has a different presentation from multiple myeloma. Patients usually have splenomegaly and/or hepatomegaly and lack osteolytic lesions. In contrast, multiple myeloma is a systemic versus localized neoplastic disorder of plasma cells that have undergone antigenic stimulation in peripheral lymph nodes and then home in on the bone marrow (the marrow produces appropriate growth factors that support growth of myeloma cells). Thus, the bone marrow is often used for diagnosis of multiple myeloma, although since the tumor is systemic, plasma cell infiltrates are frequently found in other organs (liver, spleen), particularly in cats. Monoclonal light chain may also be present in the urine in affected animals (see Bence-Jones proteinuria). - Non-neoplastic disorders: Monoclonal-like gammopathies (usually due to expansions of IgG) have been reported with occult heartworm disease, feline infectious peritonitis (rarely), Ehrlichia canis, lymphoplasmacytic enteritis, lymphoplasmacytic dermatitis and amyloidosis. These causes should ideally be ruled out before a diagnosis of multiple myeloma is made in a patient with a monoclonal-like gammopathy, particularly if on a broad base in the electrophoretogram. However, these previous reports possibly mis-interpreted a “restricted oligoclonal” gammopathy as a true monoclonal gammopathy (see the electrophoresis page for more information). Nevertheless, monoclonal expansions of IgA and IgG have been reported in 1 dog each with Ehrlichia canis and heartworm, respectively, although monoclonal peaks on electrophoresis were not obvious in either case (Jornet-Rius et al 2024).
- Increases in this fraction occur most commonly in conditions in which there is an active immune response to antigenic stimulation usually resulting in a polyclonal gammopathy. Neoplasms of immunoglobulin-producing cells (plasma cells, B-lymphocytes) can also be responsible for monoclonal increases in this fraction. For more information, see total protein electrophoresis.
- α-Globulins
Decreased globulin concentration (hypoglobulinemia)
This is usually due to decreases in gamma globulins, with decreases in other globulin fractions (alpha and beta) not always being clinically relevant.
- Artifact: We have seen low globulin concentrations as an artifact when the albumin concentration is over-estimated using the bromcresol green method (Stokol et al 2001).
- Physiologic: Globulin concentrations trend lower in young puppies and kittens (up to 8 weeks of age in both species), and calves under 6 months of age (Lumsden et al 1980, Mohri et al 2007) contributing to the lower total protein concentrations (Levy et al 2006, Rørtveit et al 2015).
- Pathologic:
- Loss: The most common cause are protein-losing enteropathies and hemorrhage with other causes of protein loss, such as exudative dermatopathies and metritis being possible. Concomitant loss of albumin in these conditions tends to maintain a normal A:G ratio with a low total protein concentration (i.e. panhypoproteinemia), although albumin concentrations appear to be more consistently decreased with hemorrhage than globulin concentrations. With inflammatory causes, such as exudative metritis, globulin concentrations can be normal due to the balancing effect of inflammation and antigenic stimulation increasing globulin concentrations.
- Decreased production: Inherited immune deficiencies in multiple immunoglobulins or immunoglobulin G may result in a low total globulin concentration (see immunoglobulin page for more information).
