Final answer:
The complement fixation test detects the presence of specific antibodies in a patient's serum by using a known antigen to see if it binds and fixes complement. If the patient's serum remains clear, indicating no hemolysis, the patient has the specific antibodies. If the serum turns pink due to hemolysis, the antibodies are absent.
Step-by-step explanation:
The complement fixation test is a laboratory method used to detect the presence of specific antibodies in a patient's serum that are bound to certain antigens. In this test, a known antigen related to a pathogen is added to the serum of Patient A or Patient B. If antibodies to that antigen are present, they will bind to the antigen and fix the complement—a group of proteins that aid in immune reactions.
This leaves no complement available to cause lysis (breakdown) of sheep red blood cells (RBCs) that are added in a later step. Therefore, if the serum turns pink, indicating hemolysis, it means the patient does not have the antibodies, as the complement was free to lyse the sheep red blood cells. Conversely, if the serum remains clear, the patient does have antibodies to the antigen because the complement was used up, or 'fixed', by the antigen-antibody reaction, thus not available to lyse the sheep red blood cells, indicating a positive test.