Hidden and classical 19S IgM rheumatoid factor in a juvenile rheumatoid arthritis patient.

Hidden 19S IgM rheumatoid factors (RF), i.e., 19S IgM RF which can be detected in the IgM containing fraction after acid gel filtration of serum, are found in 59-68% of patients with juvenile rheumatoid arthritis (JRA). Their presence generally correlates with disease activity. We describe a 12 year-old female with a polyarticular onset of JRA who, during her first 15 months of disease, was seronegative but had hidden RF titers of 1:128----1:256. Inhibition studies on her hidden RF showed specificity for HIgG greater than RIgG and equal specificity for the human IgG subclasses (IgG1 = IgG3). In the second and third year of disease, she became seropositive with RF titers varying from 1:40 to 1:320 while her hemolytic titers on her IgM fractions were decreased from 1:128 to 1:32. Inhibition studies now demonstrated a higher avidity for RIgG; and HIgG3 inhibited more than HIgG1. These studies documented for the first time a JRA patient who early in the disease was negative for RF and positive for hidden RF, and who later became seropositive.