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Complement Fixation Test in Immunology

  • In this complement fixation test in immunology post we have briefly explained about complement fixation technique for detecting the presence of antibodies in patient serum.

Complement Fixation Test in Immunology

  • Complement fixation reaction is a well-known technique for detecting the presence of antibodies in patient serum.  The complement fixation test is made up of two parts. 
  • The first component is an indicator system that combines sheep red blood cells, complement-fixing antibodies such as immunoglobulin G produced against sheep red blood cells, and an exogenous source of complement, typically guinea pig serum. When these components are combined optimally, the anti-sheep antibody binds to the surface of red blood cells. Complement then binds to the formed antigen-antibody complex, causing red blood cells to lyse.
Complement Fixation Reaction

Complement Fixation Test in Immunology: The complement pathway: Complement binds to antigen-antibody complex and leads to cell lysis. Image Source: https://bio.libretexts.org/

  • The second component is a known antigen and patient serum added to a suspension of sheep red blood cells in addition to complement. These two components of the complement fixation method are tested in sequence. Patient serum is first added to the known antigen, and complement is added to the solution. If the serum contains antibodies to the antigen, the resulting antigen-antibody complexes will bind all of the complement. 

  • The resulting antigen-antibody complexes will bind all complement if the serum contains antibody to the antigen. Sheep red blood cells and the anti-sheep antibody are then added. If complement has not been bound by an antigen-antibody complex formed from the patient serum and known antigens, it is available to bind to the indicator system of sheep cells and anti-sheep antibody. Lysis of the indicator sheep red blood cells signifies a lack of antibody in patient serum and a negative complement fixation test. 

  • If the patient’s serum does contain a complement-fixing antibody, a positive result will be indicated by the lack of red blood cell lysis. A positive result will be indicated by the lack of red blood cell lysis if the patient’s serum does contain a complement-fixing antibody. 

  • The second component is a known antigen and patient serum added to a suspension of sheep red blood cells in addition to complement. If complement has not been bound by an antigen-antibody complex formed from the patient serum and known antigens, it is available to bind to the indicator system of sheep cells and anti-sheep antibody. Lysis of the indicator sheep red blood cells signifies a lack of antibody in patient serum and a negative complement fixation test.

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