Rheumatoid arthritis is caused by the immune system mistakenly attacking the tissue lining your joints. Unlike osteoarthritis, which results from wear and tear, rheumatoid arthritis (RA) is an autoimmune disease driven by a combination of genetic susceptibility, environmental triggers, and hormonal factors. No single cause explains every case, but researchers have identified several key pieces of the puzzle.
The Immune System Turns on Your Joints
In a healthy body, the synovial membrane is a thin layer of tissue that lines your joints, producing fluid that keeps them lubricated. In RA, immune cells that normally fight infections infiltrate this membrane and treat it like a threat. A specific type of immune cell, called a T helper 17 cell, migrates into the joint and kicks off the inflammatory process. Once there, it triggers other cells in the joint lining to produce signaling proteins that recruit even more immune cells, particularly a type of white blood cell called macrophages.
These macrophages, along with the joint’s own lining cells, release a cascade of inflammatory molecules, most notably TNF-alpha, interleukin-1, and interleukin-6. These molecules drive chronic inflammation that gradually destroys cartilage and erodes bone. Over time, the inflamed tissue thickens into a mass called a pannus, which invades and damages the joint from within. This is why RA, left untreated, causes permanent joint deformity.
Genetics Load the Gun
Your genes don’t cause RA on their own, but they play a major role in determining who is vulnerable. The strongest genetic link involves a set of immune system genes called HLA-DRB1. These genes encode proteins on the surface of your cells that help the immune system distinguish “self” from “foreign.” Certain variants of HLA-DRB1, particularly those in the HLA-DR4 family, dramatically increase RA risk. Carrying the HLA-DRB1*04:01 variant, for example, is associated with roughly 4.4 times the odds of developing seropositive RA compared to someone without it.
Several related variants (HLA-DRB1*04:08, *04:05, *04:04, and *10:01) carry similarly elevated risk, while other variants appear to be protective. A large study of more than 5,000 RA patients and 15,000 controls pinpointed the risk to specific positions on the protein these genes produce. Having the right (or wrong) version of this protein changes how your immune system processes and presents proteins to T cells, making it more likely to react to your own tissues.
Genetics alone, however, account for only part of the picture. Identical twins, who share all their genes, don’t always both develop RA. Something in the environment has to pull the trigger.
Smoking Is the Strongest Environmental Trigger
Of all known environmental risk factors, cigarette smoking carries the most evidence. Smoking doesn’t just increase general inflammation. It drives a very specific biochemical process in the lungs that is central to how RA develops.
Here’s what happens: smoking significantly increases the number of immune cells in the lungs, particularly macrophages. These cells contain an enzyme that converts the amino acid arginine, found in normal proteins, into a modified form called citrulline. This process, called citrullination, changes the shape of proteins just enough that the immune system no longer recognizes them as “self.” In genetically susceptible people, the immune system produces antibodies against these citrullinated proteins, known as anti-CCP antibodies. These antibodies are the hallmark of seropositive RA and can be detected in the blood up to nine years before any joint symptoms appear.
This is why researchers believe RA may actually begin in the lungs, not the joints. The initial immune mistake happens far from where you eventually feel pain, and the disease only becomes apparent once those rogue antibodies find citrullinated proteins in the synovial tissue of your joints.
Gum Disease and the Oral Microbiome
One of the more surprising contributors to RA risk is a bacterium that causes gum disease. Porphyromonas gingivalis is the only known bacterium that produces its own version of the same enzyme responsible for citrullination. When this bacterium infects the gums, it citrullinates proteins in the surrounding tissue. In genetically susceptible people, this can trigger the same antibody response that smoking does.
Animal studies have shown this connection clearly. Mice infected with P. gingivalis developed significantly higher levels of anti-citrullinated protein antibodies and more severe arthritis than mice infected with a mutant version of the bacterium that lacked the citrullination enzyme. The citrullinated proteins produced by the bacteria appear to cross-react with human joint proteins, meaning antibodies originally made against bacterial proteins end up attacking your own joints. This helps explain the long-observed clinical link between periodontal disease and RA.
Why Women Are More Affected
Women are two to three times more likely to develop RA than men. Estrogen is known to modify immune responses, and RA disease activity often shifts during pregnancy, when estrogen levels are high. Many women experience remission during pregnancy, only to flare after delivery when hormone levels drop. The peak age of onset for RA is between 50 and 59, which for women coincides with menopause and its accompanying hormonal changes.
Interestingly, though, hormone replacement therapy after menopause does not appear to increase RA risk or worsen disease severity. A large trial that followed women for an average of 5.6 to 7.1 years found no significant difference in RA incidence between those on hormone therapy and those on placebo. This suggests that the relationship between sex hormones and RA is more complex than simple estrogen levels. Other differences in how male and female immune systems are wired likely contribute.
Obesity Creates a Pro-Inflammatory Environment
Carrying excess weight does more than stress your joints mechanically. Fat tissue actively produces signaling proteins called adipokines that interact with the immune system in ways that promote inflammation. One of the most studied is leptin, which rises with body fat and pushes the immune system toward a more aggressive, pro-inflammatory state. Leptin promotes the activation and proliferation of the same T helper 17 cells that drive RA while simultaneously suppressing regulatory T cells, the immune cells responsible for keeping autoimmune reactions in check.
Other adipokines contribute as well. Resistin, which correlates with overweight and metabolic syndrome, triggers immune cells to produce the same inflammatory molecules (TNF-alpha, IL-6, IL-1 beta) found in inflamed RA joints. In a Swedish cohort, elevated levels of the adipokine adiponectin were associated with a 70% increased risk of developing RA, independent of smoking status. Collectively, these findings help explain why obesity is highly prevalent in people with inflammatory rheumatic diseases and may contribute to their development.
Epigenetic Changes Bridge Genes and Environment
Epigenetics helps explain how environmental exposures like smoking and diet translate into lasting changes in immune function without altering your DNA sequence. Chemical modifications to your DNA and the proteins it wraps around can turn genes on or off. In RA, these modifications cause immune cells to become persistently overactive, producing inflammatory signals even after the original trigger is gone.
These epigenetic changes are reversible, which is part of what makes them significant. Diet, medications, and other environmental factors can all influence them. This means that even if you carry genetic risk factors for RA, the environmental and lifestyle factors that shape your epigenetic landscape play a real role in whether the disease develops.
Autoantibodies Appear Years Before Symptoms
One of the most striking findings in RA research is that the disease process begins long before you feel anything wrong. Anti-CCP antibodies have been detected in blood samples taken up to nine years before the onset of clinical symptoms. In someone with a family history of RA (two or fewer first-degree relatives with the disease), the presence of these antibodies indicates roughly a 69% chance of developing RA within five years.
This preclinical window matters because it means RA doesn’t strike suddenly. The immune system gradually builds a misguided response over years, likely shaped by ongoing exposure to triggers like smoking, gum disease, or other inflammatory insults. By the time joint swelling and morning stiffness appear, the autoimmune process is well established. This is one reason early detection and early treatment lead to significantly better outcomes: the goal is to intervene before irreversible joint damage occurs.