Rheumatoid serum factors are present in greater or lesser concentration in 10–40% of individuals, the titre distribution varying in different populations and being higher, as a rule, in the older age groups. Concordance between different serum factors occurs more often than would be expected by chance, and individuals with more than one factor or with a high titre of a single factor are more likely to have rheumatoid arthritis (RA).
Rheumatoid serum factors are found more often in the offspring of seropositive individuals than in persons of the same age distribution in the remainder of the population. Rheumatoid arthritis is aggregated in families independently of the rheumatoid serum factors. The aggregation is only of the severe grades with marked erosion of cartilage and bone.
It is suggested that rheumatoid serum factors may be produced by a number of different antigens and that a summation of the effect of several antigens may be important. Evidence is produced suggesting that rheumatoid arthritis may be triggered by some common respiratory infections. The more severe forms, however, appear to depend on a predisposition which may be genetically determined.