Mycoplasma pneumoniae causes community-acquired pneumonia (CAP) particularly in younger adults. The hallmark cold agglutinin test becomes positive because M. pneumoniae infection induces antibodies that also agglutinate human O-group red blood cells at 4°C. These antibodies are:

• A IgG antibodies directed against M. pneumoniae P1 adhesin protein
• B IgA antibodies directed against Mycoplasma membrane lipids
• C IgM antibodies directed against the I-antigen on red blood cell surface ✓
• D IgE antibodies that cross-react with bronchial epithelial antigens

Explanation

Cold agglutinins in Mycoplasma pneumoniae infection are polyclonal IgM antibodies that develop as an immune response to M. pneumoniae but cross-react with the I-antigen (sialylated oligosaccharide) on adult human RBCs, causing agglutination at 4°C that reverses at 37°C. Cold agglutinins are positive in ~50–70% of M. pneumoniae pneumonia (titre ≥1:128 considered significant). They are NOT specific and can be seen in EBV mononucleosis. P1 adhesin is important for pathogenesis but not the cold agglutinin target.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.