Indicator System
Definition
In the complement fixation test, sheep red blood cells pre-coated with anti-sheep-RBC antibody (sensitised cells). These cells are lysed by free complement but remain intact if complement has already been fixed by a positive reaction in the first stage. Haemolysis indicates negative; no haemolysis indicates positive.
- Test
- Complement fixation test
- Components
- Sheep red blood cells sensitised with anti-sheep-RBC antibody
- Haemolysis
- Indicates negative reaction
- No haemolysis
- Indicates positive reaction
Common questions
Why is a lack of visible haemolysis interpreted as a positive result in this test?+
Complement is a fixed, limited resource in the reaction mixture, so if it has already been consumed by a positive antigen-antibody reaction in the first stage, none remains to lyse the sensitised sheep cells added afterward, and intact cells signal that the first reaction occurred.
What causes a false negative in complement fixation testing using this indicator system?+
Anticomplementary activity in the test serum, such as bacterial contamination or improper heat inactivation, can consume complement independently of any true antigen-antibody reaction, producing a false absence of haemolysis that mimics a positive result.
Related terms
- Complement
- A group of heat-labile serum proteins (approximately 30 proteins, designated C1 through C9 in the classical pathway) that can be activated by...
- Complement Fixation
- The binding and consumption of complement proteins by an antigen-antibody immune complex. Once fixed, complement is no longer available for the lytic...
- Cross-Reactivity
- The capacity of an antibody raised against one analyte to bind structurally related compounds. In RIA, cross-reactivity is the main driver of...
- Postzone Effect
- A false-negative serological result caused by antigen excess. When antigen greatly exceeds antibody, both binding sites on each antibody molecule are occupied...
- Prozone Effect
- A false-negative serological result caused by antibody excess. When antibody concentration is too high relative to antigen, antibodies saturate individual antigen molecules...