Plasma cells in the synovial membrane and germinal center cells and internodular plasma cells in lymph nodes are the sites of origin of rheumatoid factor in active rheumatoid arthritis, whether occurring in adults or children. Plasma cells and germinal center cells which form rheumatoid factor detectable with fluorescent immune complex are less numerous than those which contain factor demonstrable with fluorescent aggregate. In the same tissues, plasma cells and germinal center cells which contain macroglobulin (19S human -globulin) detectable with fluorescent antibody—but not showing the reactivity of rheumatoid factor—are more abundant than those containing rheumatoid factor.

While macroglobulin and rheumatoid factor are almost exclusively formed in the cytoplasm, these proteins are also detectable in the nucleus of an occasional plasma cell.

Normal and pathological synovial and capsular tissues, lymph nodes, and connective tissues obtained from individuals without rheumatoid arthritis are not stained with fluorescent immune complex or, except for an unusual example of Waldenstrom's macroglobulinemia, with fluorescent aggregate.

The cellular origin, as well as certain chemical and immunological attributes, of rheumatoid factor suggests an antibody-like nature and function. The observations cited are consistent with the behavior anticipated for cellular rheumatoid factor, were it primarily an antibody direct to an altered human -globulin and cross-reacting with rabbit -globulin. However, it is also possible that there are two or more cellular rheumatoid factors.

Lesion-associated protein precipitates having the composition anticipated for rheumatoid factor-antigen complex are localized in the amyloid depositions in kidney and spleen of an individual who died with amyloidosis secondary to rheumatoid arthritis.