Mycoplasma pneumoniae causes atypical pneumonia. A specific serology test uses the patient's serum to agglutinate human group O erythrocytes at 4°C. What are these antibodies called and what antigen do they recognize?
- A Heterophile antibodies — IgM against Paul-Bunnell antigen; diagnostic of EBV infection
- B Weil-Felix antibodies — IgG against OX-19 antigen; diagnostic of rickettsial infection
- C Donath-Landsteiner antibodies — IgG against P antigen; diagnostic of paroxysmal cold hemoglobinuria
- D Cold agglutinins — IgM antibodies against I antigen on red blood cells; titers ≥1:64 support M. pneumoniae diagnosis ✓
Explanation
Cold agglutinins in M. pneumoniae infection are IgM antibodies directed against the I antigen (a polysaccharide antigen) on human group O red blood cells. They agglutinate RBCs maximally at 4°C and dissociate on warming to 37°C. Titer ≥1:64 or a fourfold rise supports M. pneumoniae diagnosis but is non-specific (also positive in EBV, CMV, lymphoma). They can cause autoimmune hemolytic anemia. Heterophile antibodies (Paul-Bunnell) diagnose EBV infectious mononucleosis. Donath-Landsteiner IgG causes PCH. Weil-Felix uses Proteus OX antigens for rickettsial diagnosis.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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