The Complement Fixation Test and Neutralization Tests

Bacteria, red blood cells, or other cells may sometimes lyse as the result of complement activity. Recall that complement is a complex system of proteins; they interact with antibody bound to antigenic components of cells (a process known as complement fixation) and cause the cells to lyse. This phenomenon is the basis for the complement fixation test, which is used to test for specific antibodies in a patient's serum. The test involves several steps (figure 17.12). For example, to test for antibodies against a specific virus:

1. The patient's serum sample is heated to inactivate any complement present.

2. The heat-inactivated serum is serially diluted, known virus is added, a known amount of animal complement is

436 Chapter 17 Applications of Immune Responses

Positive Complement-Fixation Reaction

Test system: Step 1: Test serum containing specific antibodies is heated to inactivate any complement present.

Result: No Hemolysis

Step 2: Known antigen is added, along with active animal complement.

Step 3: Indicator system is added to detect any free complement:

Red blood cells + antibodies specific for red blood cells.

Ag-Ab-C complex

Red cell-Ab complex, but no C left to lyse red cells because C was fixed by test system.

Clear supernatant

Red blood cells clumped and settled in bottom of tube

Result: Hemolysis

Negative Complement-Fixation Reaction

Test system:

Antigen + heated test serum lacking specific antibodies + unheated serum as a source of complement.

Indicator system added: Red blood cells + antibodies specific for the red cells.

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