Mycoplasma pneumoniae infection in a 20-year-old college student is confirmed by cold agglutinin test (titer 1:128). Cold agglutinins in M. pneumoniae infection are directed against:
- A P1 adhesin protein of M. pneumoniae — antibodies cross-react with RBC Forssman antigen
- B Neuraminidase-modified RBC sialic acid — enzymatic alteration exposes the cold agglutinin target
- C I antigen of human red blood cells — auto-antibodies (IgM) cross-reactive with M. pneumoniae glycolipids ✓
- D M antigen of streptococcal origin — cross-reactive with M. pneumoniae CARDS toxin
Explanation
Cold agglutinins in M. pneumoniae infection are IgM autoantibodies directed against the I antigen on adult human red blood cells. They arise as a cross-reactive immune response — M. pneumoniae glycolipid antigens share structural similarity with the I blood group antigen (a branched oligosaccharide on RBC membrane glycoproteins). These antibodies agglutinate RBCs at 4°C but dissociate at 37°C (hence 'cold' agglutinins). Clinically, they can cause cold hemagglutinin disease (autoimmune hemolytic anemia in severe M. pneumoniae infections). Cold agglutinin titer ≥1:64 supports M. pneumoniae diagnosis but lacks specificity. Treatment: azithromycin (first-line for atypical pneumonia) or doxycycline.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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Written and medically reviewed by the StethoPrep medical team.