Complement
Definition
A group of heat-labile serum proteins (approximately 30 proteins, designated C1 through C9 in the classical pathway) that can be activated by immune complexes. When activated sequentially, they form a membrane attack complex that lyses target cells. In complement fixation tests, complement is supplied in a measured, limiting amount.
- Proteins
- Roughly 30 heat-labile serum proteins, classical pathway C1 to C9
- Function
- Forms membrane attack complex that lyses target cells
- Forensic use
- Limiting reagent in complement fixation antigen-antibody tests
- Property
- Heat-labile: destroyed by heating serum to about 56C
Common questions
Why must complement be heat-labile for the fixation test to work?+
Heat-lability lets technicians deliberately inactivate a serum sample's own complement by controlled heating, then add back a known, measured amount of fresh complement, so the test result depends only on the antigen-antibody reaction being studied and not on unpredictable residual complement activity.
What does it mean when complement is 'fixed' in a test?+
Fixation means the complement has been consumed by binding to an antigen-antibody complex already formed in the sample, leaving none available to lyse indicator red blood cells added afterward, so no lysis signals a positive reaction for the antibody or antigen being tested.
Is complement fixation still used in modern forensic serology?+
It has largely been superseded by more sensitive and specific immunoassay and DNA-based methods for casework, though the underlying antigen-antibody chemistry it demonstrates remains foundational to how forensic immunology tests are understood.
Related terms
- 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...
- Conditional Probability
- The probability of event A given that event B has already occurred, written P(A|B). Conditional probability is the basis of the multiplication...
- 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...
- Event
- Any subset of the sample space to which a probability is assigned. An event may be a single outcome (this particular allele)...
- Independent Events
- Two events where the occurrence of one provides no information about the other: P(A and B) = P(A) x P(B). In forensic...
- 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...
- Mutually Exclusive Events
- Two events that cannot both occur in the same trial. If a fibre is classified as cotton, it cannot simultaneously be classified...
- 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...
- Sample Space
- The set of all possible outcomes of an experiment. In forensic genetics, the sample space for a single allele call is the...
Explained in these topics
- Basic Probability RulesThe complement of event A is the event that A does not occur. Its probability is 1 - P(A). In court, practitioners frequently present the complement of a match...
- Complement Fixation and Mixed Antigen-Antibody Reactions