Neutralization Reactions

In neutralization reactions, the harmful effects of a bacterial exotoxin or virus are eliminated by a specific antibody.

An antitoxin is an antibody produced in response to a bacterial exotoxin or a toxoid that neutralizes the exotoxin.

In a virus neutralization test, the presence of antibodies against a virus can be detected by the antibodies’ ability to prevent cytopathic effects of viruses in cell cultures.

Antibodies against certain viruses can be detected by their ability to interfere with viral hemagglutination in viral hemagglutination inhibition tests.

Complement-Fixation Reactions

Complement-fixation reactions are serological tests based on the depletion of a fixed amount of complement in the presence of an antigen- antibody reaction.

Good for detecting very small amounts of antibody, when the amount of antibody is too low to cause a precipitation or agglutination reaction.

Complement fixation was once the basis of the Wasserman test, a test to diagnose syphylis. It is still used to diagnose some viral, fungal, and rickettsial diseases.

There are two steps, the complement fixation step and the indicator step.

Complement Fixation Step

Add antigen and complement to serum. If the serum contains antibodies against the antigen they will bind to the antigen and fix the complement.

This ties up all the free complement so it can't participate in the next step, the indicator step.

Indicator Step

Add sheep red blood cells and anti-sheep red blood cell antibodies to the serum. Antibodies to the sheep red blood cells bind and can fix complement, if any is available.

If complement is available it will be fixed by the sheep red blood cell antigen-antibody complex and the sheep red blood cells will be lysed. This indicates that the serum did not contain antibodies against the antigen added in the complement fixation step and complement remained free.

If no complement is available the sheep red blood cells will not be lysed. This indicates there were antibodies against the antigen added in the complement fixation step and all the complement was tied up when it was fixed by the original antigen-antibody complex.