Complement fixation tests contain which of the following two systems?

The correct choice identifies the key components necessary for conducting complement fixation tests. The test system involves two critical elements: the test system itself, which includes the antigen and the patient's serum, and the indicator system, which usually encompasses red blood cells and complement.

In the context of these tests, the purpose of the test system is to determine if the antibodies are present in the patient's serum that can bind to the specific antigen, and thus utilize the complement system. When the patient's antibodies bind to the antigen, they activate the complement system, which then leads to a reaction that can be visually monitored through the indicator system. The indicator component typically involves a species of red blood cells that will lyse (break apart) if complement is available, which happens when the antibodies are present and have fixed the complement initially.

This structured approach is crucial for accurately interpreting the results, as it allows for the detection of specific antibodies linked to various infections or immune responses, making it central to immunological diagnostics. The other systems mentioned, while relevant to laboratory work, do not specifically define the unique framework of a complement fixation test.