Swedish scientists report the finding in Arthritis & Rheumatism.
They compared the genes and smoking habits of 930 people with RA and 383 without RA. All participants were Swedish and had similar backgrounds.
RA was most common among smokers with certain genes and antibodies that have been linked to RA. Antibodies are the immune system's tools.
Smokers with two copies of those genes -- called the HLA-DR SE genes -- were 21 times more likely to develop RA than nonsmokers without any copies of those genes.
The researchers included Lars Klareskog, MD, PhD, of the Karolinska Institut and Karolinska University Hospital in Stockholm, Sweden.
RA Risks From Smoking
Smoking was already known to raise the odds of developing RA. But, not everyone with RA smokes, and not all smokers have RA.
Why are some smokers more vulnerable to RA? No one knows for sure. It may be a mix of genetic and environmental factors, note Klareskog and colleagues.
Possibly, the genes and tobacco smoke provoke the immune system, eventually causing RA, they write.
The immune system is supposed to protect the body. But in RA, it attacks the body instead of defending it.
The Swedish study linked the HLA-DR SE genes to a certain type of antibody that's been linked to RA. The occurrence of anticitrulline antibodies is seen in almost two-thirds of people with RA and rarely in those without the disease, the researchers write.
Figuring out how smoking and genes team up to affect RA will take more work. Meanwhile, Klareskog's study is one more reason to quit smoking for good.