Which substance is used to sensitize RBCs in the complement fixation test?

In the complement fixation test, the substance used to sensitize red blood cells (RBCs) is anti-sheep hemolysin. This is because sheep red blood cells are commonly used as indicator cells in this assay. The test relies on the ability of specific antibodies in the patient's serum to bind to corresponding antigens, leading to the activation of the complement system. If the patient's serum contains the relevant antibodies, they will bind to the sheep RBCs and initiate complement fixation, causing the lysis of those sensitized red blood cells. The degree of hemolysis indicates the presence and level of antibodies in the serum being tested.

Using anti-sheep hemolysin is crucial in this context because it specifically interacts with sheep RBCs, facilitating the visualization of the complement fixation process. Each of the other hemolysins listed does not serve this specific role with sheep red blood cells, and thus they are not utilized in this particular test setup.