Explain the basis for the complement fixation test.

The complement fixation test is used to evaluate the presence of an antibody in the serum. The test is based on two components. The first component functions as an indicator system that uses a mixture of sheep erythrocytes, complement-fixing antibodies secreted against the sheep erythrocytes (immunoglobin G), and an exogenous complement source (serum of a guinea pig). When these constituents are combined under optimum conditions, the anti-sheep antibody binds on the surface of erythrocytes. The exogenous complement binds to the resultant antigen-antibody complex, therefore, bringing about lysis of the erythrocytes. The second component constitutes a known antigen and patient serum added to the compliment and suspension of sheep erythrocytes. When the elements of the two components are added procedurally, the final observation, which is the lysis or lack of lysis of erythrocytes, will signify the presence or absence of a compliment fixing antibody.