A patient with walking pneumonia (atypical pneumonia) has a positive cold agglutinin test at a titre of 1:256. Cold agglutinins in Mycoplasma pneumoniae pneumonia are IgM antibodies directed against which antigen?

• A Mycoplasma P1 adhesin protein
• B Forssman antigen on sheep RBCs
• C I antigen on human red blood cells ✓
• D Lipopolysaccharide on M. pneumoniae

Explanation

Cold agglutinins in Mycoplasma pneumoniae infection are non-specific IgM antibodies that agglutinate human erythrocytes at 4°C (cold temperature) and are directed against the I antigen on the surface of human red blood cells, not against Mycoplasma antigens. This cross-reaction arises due to molecular mimicry. A titre ≥1:32 is considered significant. Cold agglutinins are present in ~50% of M. pneumoniae pneumonia cases. Specific diagnosis uses M. pneumoniae IgM ELISA or PCR. Forssman antigen is involved in heterophil antibodies in EBV infectious mononucleosis.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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