Rheumatoid Factor IgA

Immunoglobulins which bind to the Fc region of IgG (rheumatoid factors) are characteristically present in the serum of rheumatoid arthritis (RA) patients, although they may also be found at lower frequency in certain other diseases. Initial tests for rheumatoid factor (RF) were based on the agglutination of IgG-coated red blood cells or latex particles, but because these methods detect predominantly IgM RF, the presence of rheumatoid factors of other classes (which are much poorer agglutinins) was not recognised for some time. IgG, IgA, IgD and IgE rheumatoid factors have now been identified.

The presence of IgA RF in the serum of rheumatoid arthritis (RA) patients was first shown by immunoelectrophoresis of serum proteins eluted from insoluble IgG. Although usually found in association with IgM RF, IgA RF can occur in seronegative patients. Its frequency in RA has been estimated at 56-88%, with several reports of high incidence in patients with Sjogren’s syndrome. Systemic Lupus Erythematosus (SLE) and bacterial endocarditis patients can also possess IgA RF.

In a prospective study of early RA patients, raised IgA RF levels were found to be significantly associated with the later development of erosive disease. Consequently, those patients with high levels received more treatment with specific drugs. A similar relationship between IgA RF and erosions in hands was found in a retrospective study which has lead to the suggestion that IgA RF may be related to the development of erosions. In contrast there is no such association with IgM RF levels. Thus a test capable of determining IgA RF levels can be useful in patient prognosis and management.