In a complement fixation test (CFT), a positive test result (patient serum has specific antibodies) shows:
- A Haemolysis of sensitized sheep RBCs — positive test
- B No haemolysis of sensitized sheep RBCs — complement consumed by antigen-antibody reaction ✓
- C Agglutination of sheep RBCs directly by patient antibody
- D Colour change due to enzyme-linked secondary antibody
Correct answer: B. No haemolysis of sensitized sheep RBCs — complement consumed by antigen-antibody reaction
Explanation
In CFT, if specific antibody is present, it fixes complement during the first step (antigen-antibody-complement reaction); in the indicator system, no free complement remains to lyse sensitized sheep RBCs — so NO haemolysis = positive result. Absence of specific antibody leaves complement free to lyse the RBCs (haemolysis = negative test). This 'paradox of CFT' is commonly tested. Colour change is the ELISA principle. Direct agglutination is unrelated to CFT.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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