Complement Fixation

Also known as: CFT — Complement Fixation Test

Introduction

Complement fixation is a test that exploits the fact that antibody-antigen complexes are able to activate the complement system, using the mechanism to show the presence of a specific antibody in a serum sample.

Method

A mixture of antibody, complement and antigen is incubated and an indicator system (normally antibody-coated sheep red blood cells) is added.

• As the complement reaction with the immune complex produces no visible result, sheep red blood cells are coated with anti-sheep red blood cell antibody. The complement in the mixture will react with this antibody and lyse the cells.

An indication of a positive test is the absence of lysis of the red blood cells, as the complement has already been used up by the antigen-antibody system. Controls are included to ensure that none of the reagents have taken up the complement non-specifically (e.g. contaminated serum)

Applications

• Wasserman reaction- used in the diagnosis of syphilis, the test consists of a mixture of Wasserman antigen, dilutions of the patients serum and complement (normally sourced from guinea pigs).
• Virus detection- tissue samples are innoculated with blood or tissue samples from a patient and tested using complement fixation