Rheumatoid factor isotypes following immunization.

Serum from 422 healthy subjects prior to and four weeks after multiple immunizations was studied for rheumatoid factors (RF) by an enzyme-linked immunosorbent assay (ELISA). Increase in IgM RF was detected in 22% of subjects by ELISA, but was not detectable by a standard sheep-cell agglutination test. Immunization may be a cause of RF positivity by a sensitive assay such as ELISA; it is not likely to be detected by standard clinical methods.