Rheumatoid factor (RF) antibodies are macroglobulines which appear in the serum of dogs suffering from Rheumatoid Arthritis (RA). This is frequently characterised by chronic and progressive multi-site lameness, joint swelling and joint destruction. These macroglobulines can be of the IgM, IgA or IgG subclass (mainly the IgM class). The majority of these RF antibodies are directed against the Fc part of IgG raised levels of antibodies can be found in most patients with RA (70%), but also in the other connective tissue diseases, malignancy, chronic infections and even small percentage (< 2%) in the normal population.
It is not known why patients with RA produce increased amounts of RF, but RF complexes are thought to have a role in the propagation of the RA by intra articular activation of various inflammatory factors mechanism. This can lead to inflammation with serious destructive changes of different joints caused by lysomal destruction. IgA RF was found, to be significantly associated with later development of erosive bone disease; IgA and IgG RF levels increased precede clinical manifestations. The IgA and IgG RF levels also correlate better with the Erythrocyte sedimentation rate (ESR) and elevated levels of C reactive protein. IgG RF is important due to the property of self-association leading to production of complexes without bacterial/viral antigenic stimulus.
Diagnostic criteria for RA according to the American Rheumatoid Association:
1) Morning stiffness.
2) Pain on motion (at least one point).
3) Swelling (fluid/bone)
4) Symmetric joint swelling.
5) Subcutaneous nodules over bony prominences.
6) X-ray changes typical for RA.
7) Positive RF test, by a method which has not been positive (< 5%) of normal controls.
8) Pour mucin precipitate from synovial fluid.
9) Characteristic histologic changes in synovial membrane
10) Characteristic histologic changes in nodules showing granlomatous foci.