Section 3
Immunology

SITUATION: A patient with RA has acute pneumonia but a negative throat culture. The physician suspects an infection with Mycoplasma pneumoniae and requests an IgM-specific antibody test. The test is performed directly on serial dilutions of serum less than 4 hours old. The result is positive, giving a titer of 1:32. However, the test is repeated 3 weeks later, and the titer remains at 1:32. What best explains these results?

IgM-specific antibodies do not increase fourfold between acute and convalescent serum The results are not significant because the initial titer was not accompanied by a positive test for cold agglutinins Rheumatoid factor caused a false-positive test result Insufficient time had elapsed between measurement of acute and convalescent samples