Complement Fixation
Definition
The binding and consumption of complement proteins by an antigen-antibody immune complex. Once fixed, complement is no longer available for the lytic indicator reaction. A fixed complement reading indicates that the antigen-antibody pair under test reacted with each other.
- Mechanism
- Complement bound and consumed by an antigen-antibody complex
- Detection
- Lytic indicator system, e.g. sheep red cells plus haemolysin
- Positive result
- Fixed complement, no haemolysis in the indicator step
- Status
- Largely historical, superseded by newer immunoassays
Common questions
Why does a positive complement fixation test show no haemolysis?+
When the test antigen and antibody react, they consume the added complement, leaving none available to lyse the indicator red blood cells in the second step, so an intact, non-lysed indicator system is read as a positive reaction, which is counterintuitive on first encounter.
Why has complement fixation fallen out of routine forensic use?+
It requires careful titration of complement and reagents, is technically demanding to standardise, and has been outperformed in sensitivity and ease of use by later immunoassay formats such as ELISA, so it survives mainly in historical case review and some specialised serology contexts.
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...
- 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...
- Indicator System
- 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...
- 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...