Figure 17.12 Complement Fixation Test Procedure added, and the mixture is incubated, allowing the virus antigen and the serum antibody to interact and bind, or fix, the added complement. If no specific antibodies are present, the complement will remain free in the mixture.

3. To visualize the reaction, an indicator system is added to detect any free complement in the mixture. The indicator system usually consists of sheep red blood cells and anti-sheep-red-blood-cell antibodies.

If a specific virus antigen-antibody reaction has fixed or bound the complement (in step 2), the red cells in the indicator system will not be lysed, but will clump and settle; this is a positive test for the antiviral antibody. If complement has not been fixed, the red cells in the indicator system will be lysed, releasing their hemoglobin. This is a negative reaction for antiviral antibodies.

Once widely used, the complement fixation test is complicated and lengthy. Consequently, it has been often superseded by newer, easier techniques, such as ELISA.

Neutralization tests are another example of useful tests that are often superseded by other, easier methods. In the virus neutralization test, serum is mixed with a known viral suspension. If antibodies to that particular virus are present, they will bind to it and neutralize the virus, preventing its attachment to and subsequent infection of cells. When the virus is then added to an appropriate cell culture, it is unable to replicate and cause cell damage. Neutralizing antibodies can be demonstrated in herpesvirus infections and some other viral infections.

Neutralization tests can also be used to test for toxin. Specific antibodies against the toxin (antitoxin) will neutralize the effects of the toxin. For example, in a suspected case of botulism, a lethal form of food poisoning, leftover food thought to be the source of the toxin can be tested by injecting some of the food into mice. The antigenic type of toxin is determined by mixing part of the food sample with specific antitoxin before injection. Untreated food containing botulinum toxin kills the mice, but they are protected if the food is first mixed with specific antitoxin that neutralizes the toxin. Toxin may also be detected by indirect hemagglutination or radioimmunoassay. ■ botulism, p. 672

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