Elevated cold agglutinin titers are most characteristic of which infection?

• A. Mycoplasma pneumoniae
• B. Staphylococcus aureus
• C. Streptococcus pyogenes
• D. Legionella pneumophila

Answer: (A)

Elevated cold agglutinin titers arise from IgM antibodies that bind red blood cells at low temperatures, causing the cells to clump together and, in some cases, triggering hemolysis. This immune response is classically seen with Mycoplasma pneumoniae infection, an atypical pneumonia common in younger adults. The presence of these cold-reacting antibodies during illness is a helpful vascular clue pointing toward Mycoplasma as the cause, since these specific antibodies are not a typical feature of pneumonias caused by the other listed organisms. The other pathogens—Staphylococcus aureus, Streptococcus pyogenes, and Legionella pneumophila—do not characteristically induce cold agglutinin production, so they are less likely explanations when elevated cold agglutinin titers are present.