Complement (C3 and C4), Acute Phase Protein

 Acute-phase protein (Acute Phase Reactants)

• Acute-phase protein is raised in inflammatory conditions.
• When there is an increase in a protein called positive acute-phase protein.
  • In the case of a decrease in the acute phase protein, is called negative phase protein.
• The acute phase proteins (positive) are protein whose concentration increases in the plasma and after the disease episode is over then it decreases and may become normal.

Complement C3 and C4 (Complements)

Sample

• The venous blood of the patient is needed to prepare the serum.
• This can be done on EDTA plasma.

Precautions

• C3 is very unstable at room temperature.
• It is advised to do the test immediately.
• Or freeze at -70 °C.

Indication

1. Advised to diagnose angioedema.
2. To monitor:
   1. The patient with systemic lupus erythematosus nephritis.
   2. In post-streptococcal nephritis.
   3. In membranoproliferative nephritis.

Pathophysiology

1. Complements are a group of globulin proteins and acts as an enzyme.
   1. These enzymes facilitate immunological and inflammatory response.
2. These are heat-labile plasma proteins.
3. These consists of at least 14 protein which has a biological function in the immune system. The more common are labeled as C1 to C9. 
   1. Besides the major components (C1 to C9), there are subcomponents, and inhibitors are involved in the system.
4. The complement acts as an enzyme.
   1. This will help in the immunologic response and inflammation.
   2. They help in the chemotaxis and activation of the WBCs.
   3. There is an increase in vascular permeability, allowing the Antibody and the WBCs to be delivered at the site of the immune complex (Ag+Ab).
   4. Complement helps in the process of phagocytosis and the immune adherence of the antibody to the antigen.
   5. The complement activation leads to the lysis of the antigen or the bacteria.
5. Complement is the biological amplifier.
   1. First, there is the activation of the few molecules of C1q.
   2. In the next stage more and more molecules are activated.
   3. In the final stage, there are thousands of molecules.

6. The classical pathway is activated by the C1 and ultimately there is the activation of C1 to C9.
7. In the alternate pathway, there is a bypass of the C1q and direct activation of C3.
   1. The alternate pathway may be activated by IgA.
   2. This is activated in the absence of the antibody.
   3. Polysaccharides of the bacterial cell wall can activate C3 and leads to the destruction of the bacteria.

8. In the case of antigen and antibody complexes, the complement is consumed which leads to a decreased level of compliments.
9. Complement deficiency may be:
   1. Congenital like angioedema. There is a congenital lake of a C1 – inhibitor often called C1 esterase.
      1. The Complement system is activated, and the complement is consumed or used up which will lead to falling in the complement level.
   2. Acquired decreased level is seen in the chronic diseases where there is the presence of Ag+Ab complexes leading to overactivation of the complement system, complement is consumed and there is a fall in the level of complement e.g. serum sickness, infectious endocarditis, lupus erythematosus, vasculitis, and glomerulonephritis.

C3:

1. C3 molecular weight is 185 kD with a normal serum level of 1200 µg/mL, acute-phase serum level increases 30% and the chromosomal location is 19p13.2-13.3.
2. C3 after activation is converted into C3a and C3b.
   1. C3a is an anaphylatoxin binds to mast cells and triggers the release of histamine and other mediators’ products.
      1. This will leads to vasodilation, erythema, and swelling.
      2. This will also lead to bronchoconstriction.
      3. C3a, when injected into the skin, leads to wheel and flare.
   2. C3b attaches to the antigen-antibody complex through the C3b receptor on the inflammatory cells and leads to its adherence to the:
      1. Neutrophils.
      2. Basophils.
      3. Eosinophils.
      4. Monocytes.
      5. This will leads to enhance opsonization of the antigen-antibody complex and phagocytosis.
      6. C3b coated particles also activate B-lymphocytes to produce antibodies.
      7. In the early phase of viral infection, C3b leads to neutralization of the viruses when the antibody level is limited.
3. C3 has an important role in the classical and alternative pathways.
4. This is a major opsonin after the activation mainly by C3b.
   1. Any decrease in C3b leads to defective opsonization.
   2. In C3b deficiency, Neisserial infections are very common.

5. In the case of deficiency, there are recurrent pyogenic infections, particularly due to encapsulated bacteria.
6. When it occurs in childhood, then there is a recurrent bacterial infection, mainly by the gram-negative bacteria like:
   1. Neisseria meningitidis.
   2. Enterobacter aerogenes.
   3. Escherichia coli.
   4. Haemophilus influenzae.
7. Infection with gram-positive bacteria may also occur.
8. C3 deficiency may cause:
   1. Upper and lower respiratory infections including pneumonia.
   2. There are episodes of sinusitis.
      1. Tonsillitis.
      2. Otitis media.
   3. 26% of C3 deficiency patients may develop an autoimmune complex disease like systemic lupus erythematosus.
   4. 26% of C3 deficiency patients develop membranoproliferative or mesangiocapillary glomerulonephritis. 

C4:

1. C4 has a highly pleomorphic gene and located in the HLA complex chromosome 6.
2. C4 has a molecular weight of 200 kD, a normal serum level of 600 µg/mL, an acute serum phase increase of 34%, and chromosome number 6p21.3.
3. Partial deficiency of the C4 predisposes to SLE.
4. Deficiency of C4a and C4b are associated in:
   1. Scleroderma.
   2. IgA associated nephropathy.
   3. Diabetes mellitus.
   4. Chronic hepatitis.
   5. Membranous nephropathy.
   6. Henoch schonlein purpura.
5. Complete deficiency of the C4 is rare.

Normal

• Total complement = 75 to 160 units/mL

Source 1

C3   =  

• Cord serum = 65 to 111.8 mg/dL
  • Children = 86.6 to 135.8 mg/dL
• Adult  = 83 to 177 mg/dL

Source 2 

• C3 = 55 to 120 mg/dL

C4    =  

Source 1

• Cord serum  = 13.7 to 44.9 mg/dL
  • one month = 27.5 to47.3 mg/dL
  • 6 months = 30.5 to 58.7 mg/dL
• Adult  =  28.7 to 67.5 mg/dL

Source 2 

• C4 = 20 to 50 mg/dL

The decreased level of complement (C1 to C9) is seen in:

1. Congenital as seen in angioedema.
2. Autoimmune diseases like SLE, serum sickness, rheumatoid arthritis, Sjogren’s syndrome, and lupus nephritis.
3. Malnutrition.
4. Cirrhosis.
5. Hepatitis.
6. Acute kidney transplant rejection.
7. In the case septicemia (sepsis).

The increased level of complements (C1 to C9) is seen in:

1. Acute rheumatic fever.
2. Ulcerative colitis.
3. Acute myocardial infarction.
4. Malignant tumors.

Acute-phase proteins and their value in acute and chronic inflammation:

Acute-phase proteins	Acute Inflammation	Chronic Inflammation
Albumin	decreased	decreased
Prealbumin	decreased	decreased
CRP	increased	increased
Alpha-1-antitrypsin	increased	increased
Ceruloplasmin	increased	increased
Haptoglobin	increased	increased
Transferrin	decreased	increased
Fibrinogen	increased	increased
Complement (C3)	increased	increased

 Table showing Positive and Negative acute phase protein

Positive acute-phase proteins	Negative acute phase proteins
Ceruloplasmin	Prealbumin
Haptoglobin	Albumin
Fibrinogen	Transferrin
Complement (C 3, 4)

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