Final answer:
The complement fixation test is used to detect antibodies against specific pathogens in a patient's serum. It uses complement as a tool to determine if antibodies are present or not.
Step-by-step explanation:
The purpose of the complement fixation test is to determine whether a patient's serum contains antibodies to a specific pathogen. This test uses complement, a series of proteins in the blood that can become activated in the presence of invading microbes. If the patient's serum contains antibodies to the pathogen, complement fixation will occur, preventing the lysis of red blood cells that are added to the solution. On the other hand, if no antibodies are present, complement will be available to lyse the red cells, resulting in a negative test.