Part of
Infection & inflammation
Part of
Infection & inflammation
Blood biomarker
Part of
Infection & inflammation
Complement Component 4 (C4)
Complement Component 4 (C4)
Complement component 4 (C4) is a protein of the immune system, helping to fight infections.
Complement component 4 (C4) is a protein of the immune system, helping to fight infections.
Complement Component 4 (C4)
Normal range
Normal range
0.15-0.43 g/L
0.15-0.43 g/L
Normal range
0.15-0.43 g/L
0.15-0.43 g/L
Normal range
Complement Component 4 (C4)
Complement Component 4 (C4)
Complement component 4 (C4) is a protein of the immune system, helping to fight infections.
Complement Component 4 (C4)
Normal range
0.15-0.43 g/L
Normal range
0.15-0.43 g/L
0.15-0.43 g/L
Normal range
Complement Component 4 (C4)
Complement Component 4 (C4)
Complement component 4 (C4) is a protein of the immune system, helping to fight infections.
Complement Component 4 (C4)
Normal range
0.15-0.43 g/L
Normal range
0.15-0.43 g/L
0.15-0.43 g/L
Normal range
Complement Component 4 (C4)
Dr. Jack Devin
MBBS
A Key Player in Your Immune System's Defence Force
A Key Player in Your Immune System's Defence Force
Working in close partnership with its more abundant counterpart, C3, Complement C4 is another vital protein in your immune system's complement cascade. This system acts as a first line of defence against infection. C4 helps to kick-start the 'classical pathway' of this cascade, a specific sequence of events that is typically triggered when the body detects antibodies bound to an invader, like a bacterium or virus.
Think of C4 as a specialist unit within your immune system’s army. Its activation is a critical signal that a specific type of immune response has been launched. Measuring your blood C4 level, especially alongside C3, helps doctors understand exactly how your immune system is responding and can point towards specific autoimmune conditions or rare genetic deficiencies.
Working in close partnership with its more abundant counterpart, C3, Complement C4 is another vital protein in your immune system's complement cascade. This system acts as a first line of defence against infection. C4 helps to kick-start the 'classical pathway' of this cascade, a specific sequence of events that is typically triggered when the body detects antibodies bound to an invader, like a bacterium or virus.
Think of C4 as a specialist unit within your immune system’s army. Its activation is a critical signal that a specific type of immune response has been launched. Measuring your blood C4 level, especially alongside C3, helps doctors understand exactly how your immune system is responding and can point towards specific autoimmune conditions or rare genetic deficiencies.
High C4 Levels: A General Sign of Inflammation
High C4 Levels: A General Sign of Inflammation
High C4 Levels: A General Sign of Inflammation
Learn why, like C3, C4 levels can rise as part of the body's general inflammatory response to infection or injury.
Read more
Low C4 Levels: A Clue to Autoimmunity or Deficiency
Low C4 Levels: A Clue to Autoimmunity or Deficiency
Low C4 Levels: A Clue to Autoimmunity or Deficiency
Discover why a low C4 level is often more clinically significant, pointing towards consumption by an autoimmune disease or a rare genetic condition.
Read more
C4's Special Role: Lupus and Hereditary Angioedema
C4's Special Role: Lupus and Hereditary Angioedema
C4's Special Role: Lupus and Hereditary Angioedema
Understand C4's unique link to specific conditions like Systemic Lupus Erythematosus (SLE) and Hereditary Angioedema (HAE).
Read more
Understanding Your Results
Understanding Your Results
Understanding Your Results
Learn how doctors interpret your C4 and C3 levels together to pinpoint the nature of your immune system's activity.
Read more
High C4 Levels: A General Sign of Inflammation
Similar to C3, Complement C4 is an 'acute-phase reactant'. This means that when your body is faced with a sudden challenge, such as an infection or significant injury, the liver increases its production of C4 along with other inflammatory proteins.
An elevated C4 level can be seen in the same situations as a high C3 level, including during acute infections or in response to trauma. Conditions such as obesity, metabolic syndrome, or certain malignancies can also produce mild, sustained C4 elevations.
It is part of the body's non-specific, immediate defence mechanism designed to contain a threat. In most cases, a high C4 level is temporary and will return to the normal range once the underlying inflammatory trigger has been resolved. For this reason, doctors are often less concerned with a high C4 level than they are with a low one.
High C4 Levels: A General Sign of Inflammation
Similar to C3, Complement C4 is an 'acute-phase reactant'. This means that when your body is faced with a sudden challenge, such as an infection or significant injury, the liver increases its production of C4 along with other inflammatory proteins.
An elevated C4 level can be seen in the same situations as a high C3 level, including during acute infections or in response to trauma. Conditions such as obesity, metabolic syndrome, or certain malignancies can also produce mild, sustained C4 elevations.
It is part of the body's non-specific, immediate defence mechanism designed to contain a threat. In most cases, a high C4 level is temporary and will return to the normal range once the underlying inflammatory trigger has been resolved. For this reason, doctors are often less concerned with a high C4 level than they are with a low one.
High C4 Levels: A General Sign of Inflammation
Similar to C3, Complement C4 is an 'acute-phase reactant'. This means that when your body is faced with a sudden challenge, such as an infection or significant injury, the liver increases its production of C4 along with other inflammatory proteins.
An elevated C4 level can be seen in the same situations as a high C3 level, including during acute infections or in response to trauma. Conditions such as obesity, metabolic syndrome, or certain malignancies can also produce mild, sustained C4 elevations.
It is part of the body's non-specific, immediate defence mechanism designed to contain a threat. In most cases, a high C4 level is temporary and will return to the normal range once the underlying inflammatory trigger has been resolved. For this reason, doctors are often less concerned with a high C4 level than they are with a low one.
Low C4 Levels: A Clue to Autoimmunity or Deficiency
A low C4 level is a much more specific and often more significant finding. It suggests that the complement system's classical pathway is being heavily activated and C4 is being consumed faster than the liver can produce it. This is a strong indicator that the immune system is engaged in a specific type of battle.
The most common cause of a low C4 is an autoimmune disease, where the immune system mistakenly creates antibodies against its own tissues. These 'auto-antibody' complexes are potent activators of the classical pathway, leading to a continuous drain on C4 levels.
Less commonly, a low C4 can be due to a rare inherited genetic deficiency. Individuals with a C4 deficiency may be more susceptible to infections and have a higher risk of developing autoimmune conditions like lupus.
Low C4 Levels: A Clue to Autoimmunity or Deficiency
A low C4 level is a much more specific and often more significant finding. It suggests that the complement system's classical pathway is being heavily activated and C4 is being consumed faster than the liver can produce it. This is a strong indicator that the immune system is engaged in a specific type of battle.
The most common cause of a low C4 is an autoimmune disease, where the immune system mistakenly creates antibodies against its own tissues. These 'auto-antibody' complexes are potent activators of the classical pathway, leading to a continuous drain on C4 levels.
Less commonly, a low C4 can be due to a rare inherited genetic deficiency. Individuals with a C4 deficiency may be more susceptible to infections and have a higher risk of developing autoimmune conditions like lupus.
Low C4 Levels: A Clue to Autoimmunity or Deficiency
A low C4 level is a much more specific and often more significant finding. It suggests that the complement system's classical pathway is being heavily activated and C4 is being consumed faster than the liver can produce it. This is a strong indicator that the immune system is engaged in a specific type of battle.
The most common cause of a low C4 is an autoimmune disease, where the immune system mistakenly creates antibodies against its own tissues. These 'auto-antibody' complexes are potent activators of the classical pathway, leading to a continuous drain on C4 levels.
Less commonly, a low C4 can be due to a rare inherited genetic deficiency. Individuals with a C4 deficiency may be more susceptible to infections and have a higher risk of developing autoimmune conditions like lupus.
C4's Special Role: Lupus and Hereditary Angioedema
Low C4 levels are particularly associated with two key conditions:
Systemic Lupus Erythematosus (SLE): A low C4, often accompanied by a low C3, is a classic feature of lupus. It is one of the criteria used in diagnosis and is a valuable marker for monitoring disease activity. A drop in C4 levels in someone with known lupus can signal an impending disease flare, especially one affecting the kidneys.
Hereditary Angioedema (HAE): This is a rare but serious genetic disorder characterised by recurrent episodes of severe swelling in various parts of the body. While not an autoimmune disease, it is caused by a deficiency in another protein (C1-inhibitor) that regulates the complement system. This leads to uncontrolled activation and consumption of C4, making a low C4 level a key screening test for the condition.
C4's Special Role: Lupus and Hereditary Angioedema
Low C4 levels are particularly associated with two key conditions:
Systemic Lupus Erythematosus (SLE): A low C4, often accompanied by a low C3, is a classic feature of lupus. It is one of the criteria used in diagnosis and is a valuable marker for monitoring disease activity. A drop in C4 levels in someone with known lupus can signal an impending disease flare, especially one affecting the kidneys.
Hereditary Angioedema (HAE): This is a rare but serious genetic disorder characterised by recurrent episodes of severe swelling in various parts of the body. While not an autoimmune disease, it is caused by a deficiency in another protein (C1-inhibitor) that regulates the complement system. This leads to uncontrolled activation and consumption of C4, making a low C4 level a key screening test for the condition.
C4's Special Role: Lupus and Hereditary Angioedema
Low C4 levels are particularly associated with two key conditions:
Systemic Lupus Erythematosus (SLE): A low C4, often accompanied by a low C3, is a classic feature of lupus. It is one of the criteria used in diagnosis and is a valuable marker for monitoring disease activity. A drop in C4 levels in someone with known lupus can signal an impending disease flare, especially one affecting the kidneys.
Hereditary Angioedema (HAE): This is a rare but serious genetic disorder characterised by recurrent episodes of severe swelling in various parts of the body. While not an autoimmune disease, it is caused by a deficiency in another protein (C1-inhibitor) that regulates the complement system. This leads to uncontrolled activation and consumption of C4, making a low C4 level a key screening test for the condition.
Understanding Your Results
A C4 level is measured with a simple blood test and is almost always ordered alongside a C3 test. Interpreting the two results together gives a clearer picture of what's happening in the complement system.
Complement C4 reference ranges in grams per litre (g/L) by age range:
Ages 0-29 Normal Range: Normal: 0.15-0.43 g/L
Ages 30-39 Normal Range: Normal: 0.16-0.46 g/L
Aged 40+ Normal Range: Normal: 0.18-0.49 g/L
Understanding C4 alongside a C3 result:
Low C4 with a low C3: This pattern is highly suggestive of classical pathway activation, commonly seen in autoimmune diseases like lupus.
Low C4 with a normal C3: This can point towards an inherited C4 deficiency or may be seen in the early stages of an autoimmune process before C3 levels have dropped. It is also a feature of Hereditary Angioedema.
Normal C4 with a low C3: This suggests that the complement system is being activated through a different route (the 'alternative pathway'), which can occur in certain types of kidney disease or infections.
Understanding Your Results
A C4 level is measured with a simple blood test and is almost always ordered alongside a C3 test. Interpreting the two results together gives a clearer picture of what's happening in the complement system.
Complement C4 reference ranges in grams per litre (g/L) by age range:
Ages 0-29 Normal Range: Normal: 0.15-0.43 g/L
Ages 30-39 Normal Range: Normal: 0.16-0.46 g/L
Aged 40+ Normal Range: Normal: 0.18-0.49 g/L
Understanding C4 alongside a C3 result:
Low C4 with a low C3: This pattern is highly suggestive of classical pathway activation, commonly seen in autoimmune diseases like lupus.
Low C4 with a normal C3: This can point towards an inherited C4 deficiency or may be seen in the early stages of an autoimmune process before C3 levels have dropped. It is also a feature of Hereditary Angioedema.
Normal C4 with a low C3: This suggests that the complement system is being activated through a different route (the 'alternative pathway'), which can occur in certain types of kidney disease or infections.
Understanding Your Results
A C4 level is measured with a simple blood test and is almost always ordered alongside a C3 test. Interpreting the two results together gives a clearer picture of what's happening in the complement system.
Complement C4 reference ranges in grams per litre (g/L) by age range:
Ages 0-29 Normal Range: Normal: 0.15-0.43 g/L
Ages 30-39 Normal Range: Normal: 0.16-0.46 g/L
Aged 40+ Normal Range: Normal: 0.18-0.49 g/L
Understanding C4 alongside a C3 result:
Low C4 with a low C3: This pattern is highly suggestive of classical pathway activation, commonly seen in autoimmune diseases like lupus.
Low C4 with a normal C3: This can point towards an inherited C4 deficiency or may be seen in the early stages of an autoimmune process before C3 levels have dropped. It is also a feature of Hereditary Angioedema.
Normal C4 with a low C3: This suggests that the complement system is being activated through a different route (the 'alternative pathway'), which can occur in certain types of kidney disease or infections.
The Takeaway
C4 serves as an important regulatory component in your immune system's complement cascade, helping maintain the balance between fighting infections and preventing autoimmune damage. Abnormal levels often indicate immune system dysfunction requiring medical evaluation.
Understanding C4 helps you appreciate why complement testing is valuable in diagnosing and monitoring autoimmune diseases, particularly when used alongside other immune markers and clinical assessment.
Don't ignore symptoms that might suggest autoimmune activity, such as joint pain, skin rashes, or kidney problems, especially if accompanied by abnormal C4 levels, as early evaluation and treatment can prevent further complications.
The Takeaway
C4 serves as an important regulatory component in your immune system's complement cascade, helping maintain the balance between fighting infections and preventing autoimmune damage. Abnormal levels often indicate immune system dysfunction requiring medical evaluation.
Understanding C4 helps you appreciate why complement testing is valuable in diagnosing and monitoring autoimmune diseases, particularly when used alongside other immune markers and clinical assessment.
Don't ignore symptoms that might suggest autoimmune activity, such as joint pain, skin rashes, or kidney problems, especially if accompanied by abnormal C4 levels, as early evaluation and treatment can prevent further complications.
The Takeaway
C4 serves as an important regulatory component in your immune system's complement cascade, helping maintain the balance between fighting infections and preventing autoimmune damage. Abnormal levels often indicate immune system dysfunction requiring medical evaluation.
Understanding C4 helps you appreciate why complement testing is valuable in diagnosing and monitoring autoimmune diseases, particularly when used alongside other immune markers and clinical assessment.
Don't ignore symptoms that might suggest autoimmune activity, such as joint pain, skin rashes, or kidney problems, especially if accompanied by abnormal C4 levels, as early evaluation and treatment can prevent further complications.
References
British Society for Immunology. (2020). Complement system disorders: diagnosis and management. Clinical Immunology Guidelines.
Lupus UK. Tests for Lupus.
Cicardi, M., & Zanichelli, A. (2010). The pathophysiology of hereditary angioedema. International immunopharmacology, 10(12), 1441-1446.
References
British Society for Immunology. (2020). Complement system disorders: diagnosis and management. Clinical Immunology Guidelines.
Lupus UK. Tests for Lupus.
Cicardi, M., & Zanichelli, A. (2010). The pathophysiology of hereditary angioedema. International immunopharmacology, 10(12), 1441-1446.
References
British Society for Immunology. (2020). Complement system disorders: diagnosis and management. Clinical Immunology Guidelines.
Lupus UK. Tests for Lupus.
Cicardi, M., & Zanichelli, A. (2010). The pathophysiology of hereditary angioedema. International immunopharmacology, 10(12), 1441-1446.
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