In rheumatoid arthritis, the serologically detectable antibody is primarily of the class:

In rheumatoid arthritis, the serologically detectable antibody is primarily of the class:

• A. IgA
• B. IgE
• C. IgG
• D. IgM

Answer: (D)

Rheumatoid factor is the autoimmune antibody most classically associated with rheumatoid arthritis, and the majority of RF detected in RA is of the IgM class. IgM is a pentamer, which makes it particularly effective at cross-linking IgG molecules to form immune complexes. These RF-IgM–IgG complexes activate complement and deposit in the synovium, driving inflammation and joint damage that characterize RA. While other antibodies can be present—in particular, anti-CCP antibodies are often IgG—the common serologic marker tested for RA is IgM rheumatoid factor.

Rheumatoid factor is the autoimmune antibody most classically associated with rheumatoid arthritis, and the majority of RF detected in RA is of the IgM class. IgM is a pentamer, which makes it particularly effective at cross-linking IgG molecules to form immune complexes. These RF-IgM–IgG complexes activate complement and deposit in the synovium, driving inflammation and joint damage that characterize RA. While other antibodies can be present—in particular, anti-CCP antibodies are often IgG—the common serologic marker tested for RA is IgM rheumatoid factor.