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 Acute phase protein (Acute Phase Reactants)

Complement C3 and C4 (Complements)




  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.


  1. Complements are a group of globulin proteins.
  2. These are heat labile plasma proteins.
  3. These consists of at least 14 protein which has a biological function in the immune system.
  4. These 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. The complement activation leads to lysis of the antigen or the bacteria.
  5. Complement is the biological amplifier.
    1. First, there is 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.

  1. The classical pathway is activated by the C1 and ultimately there is activation of C1 to C9.
  2. 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 destruction of the bacteria.

  1. In the case of antigen and antibody complexes, the complement is consumed which leads to decreased level of compliments.


C3   =  

C4    =  

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 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     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)  

Possible References Used

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