A 19-year-old student has fever, dry cough and bilateral interstitial pneumonia. Cold agglutinin titre performed on his serum shows agglutination of human O group red cells at 4 degrees Celsius at a dilution of 1:64, which reverses on warming. The cold agglutinins in this infection are directed against which antigen?

• A H antigen of Salmonella
• B Forssman antigen
• C P1 antigen of parainfluenza virus
• D I antigen on adult erythrocytes ✓

Explanation

Primary atypical pneumonia due to Mycoplasma pneumoniae induces IgM autoantibodies directed against the I antigen of human erythrocytes. These cold agglutinins bind red cells at low temperature (4 degrees Celsius) and dissociate on warming to 37 degrees Celsius, a feature that distinguishes them from other agglutinins. D titre of 1:32 or higher is considered significant. Forssman heterophile antibodies are seen in infectious mononucleosis detected by the Paul-Bunnell test, not in mycoplasmal infection.

Reference: Ananthanarayan and Paniker Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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