Rheumatoid arthritis (RA) has no single cause. It develops when a combination of genetic susceptibility, hormonal factors, and environmental triggers pushes the immune system to attack the tissue lining your joints. The disease affects roughly 0.5 to 1% of the global population, and women are two to three times more likely to develop it than men. Understanding what drives this process can help explain why some people develop RA while others with similar risk factors never do.
How the Immune System Attacks Your Joints
In a healthy joint, the synovial membrane is a thin tissue that produces fluid to keep things lubricated. In RA, the immune system floods this membrane with white blood cells, particularly macrophages and T cells, that release a cascade of inflammatory signals. These signals recruit even more immune cells, creating a self-reinforcing cycle of inflammation.
The inflammatory signals that do the most damage include a group of proteins that promote tissue destruction in several ways. Some drive the cartilage cells in your joints to self-destruct. Others stimulate the formation of osteoclasts, the cells responsible for breaking down bone. Still others promote the growth of new blood vessels into the inflamed tissue, feeding its expansion into a thick, aggressive layer called pannus that physically invades cartilage and bone.
At the same time, cells within the synovial lining begin to behave like tumor cells. They lose the normal growth controls that keep tissue in check and multiply unchecked, producing enzymes that digest the structural proteins holding cartilage together. This combination of immune attack from above and invasive tissue growth from within is what makes RA so destructive when left untreated.
Genetic Risk Factors
Genetics account for a significant portion of RA risk, with estimates of heritability ranging from 40 to 65% depending on the study. The strongest genetic link involves a group of immune system genes called HLA-DRB1. Several specific variants of this gene, collectively known as the “shared epitope,” are found at much higher rates in people with RA. These gene variants shape how your immune cells present proteins to the rest of the immune system, and certain versions appear to make the immune system more likely to misidentify normal body proteins as threats.
Outside the HLA region, the most important genetic contributor is a gene called PTPN22, which encodes a protein involved in regulating immune cell activation. A specific mutation in this gene has been repeatedly linked to RA risk. It essentially loosens the brakes on immune cells, making them more reactive. Dozens of other genes with smaller individual effects have also been identified, most of them involved in immune regulation. But having these genes doesn’t guarantee you’ll develop RA. They load the gun; something else pulls the trigger.
Why Women Are More Affected
The 3-to-1 female-to-male ratio in RA points strongly to hormonal involvement, though the relationship is more complex than it first appears. Declining levels of estrogen and progesterone, whether during menopause or in the months after giving birth, appear to increase both the risk and severity of RA. This seems counterintuitive since estrogen is often thought of as anti-inflammatory, but the reality is more nuanced.
One key mechanism involves a protein called AIRE that trains the immune system during development to recognize the body’s own tissues as “self” rather than foreign. After puberty, women express lower levels of AIRE than men, and estrogen itself appears to suppress AIRE production. This means the female immune system may be slightly less thorough at weeding out self-reactive immune cells, the very cells that drive autoimmune diseases like RA.
Androgens (male hormones) seem to offer some protection in the other direction. Genetic variants that lead to higher androgen production from precursor hormones are associated with reduced RA susceptibility in women. This protective effect of androgens, combined with the complex and sometimes paradoxical effects of estrogen, helps explain the persistent sex difference in RA rates.
Smoking and Protein Citrullination
Smoking is the single most well-established environmental risk factor for RA. The mechanism centers on a chemical modification called citrullination, in which the amino acid arginine in proteins is converted to citrulline. This subtly changes the shape of the protein, and in genetically susceptible people, the immune system treats these modified proteins as foreign invaders.
The body then produces antibodies against citrullinated proteins (called ACPAs), which can be detected in the blood years before RA symptoms appear. These antibodies bind to citrullinated proteins in joint tissue and activate macrophages to produce inflammatory signals, particularly TNF-alpha, one of the key drivers of joint destruction. Smoking both promotes the citrullination process and worsens disease progression once RA has developed.
Oral and Gut Bacteria
The connection between gum disease and RA has been observed for decades, and research now offers a clear biological explanation. A bacterium called Porphyromonas gingivalis, commonly found in periodontal disease, produces its own citrullination enzyme. This enzyme can modify both bacterial and human proteins, triggering the same anti-citrullinated protein antibody response that smoking promotes. In one study, 77% of RA patients showed immune responses to peptides derived from this bacterial enzyme, and about a third had elevated antibodies to it before their RA symptoms even began.
Another oral bacterium, Aggregatibacter actinomycetemcomitans, takes a different path to the same result. Rather than citrullinating proteins directly, it forces human immune cells called neutrophils to ramp up their own citrullination enzymes, promoting the modification of the body’s own proteins.
The gut microbiome plays a parallel role. People in the preclinical stages of RA, before joint symptoms appear, show increased levels of Prevotella species in their intestines. These bacteria stimulate immune cells to produce inflammatory signals that promote a type of immune response strongly linked to autoimmunity. Another gut bacterium, Collinsella aerofaciens, is significantly elevated in early RA and may compromise the integrity of the gut lining, allowing bacterial products to enter the bloodstream and trigger systemic inflammation. Conversely, bacteria that produce butyrate, a short-chain fatty acid important for gut health, appear protective. Their abundance is inversely correlated with ACPA levels and the number of affected joints.
Obesity and Fat-Derived Inflammation
Excess body fat, particularly visceral fat around the organs, contributes to RA risk through signaling molecules called adipokines that fat tissue releases into the bloodstream. In obesity, adipokine production shifts toward a pro-inflammatory profile that can affect joints directly.
Leptin, one of the best-studied adipokines, stimulates the synovial lining cells in joints to produce inflammatory chemicals and enzymes that degrade cartilage. It also promotes the migration of these cells and the growth of new blood vessels, both hallmarks of RA joint damage. Adiponectin, despite being generally considered beneficial in metabolic health, appears to play a harmful role in RA joints by promoting inflammation, stimulating blood vessel growth in the synovium, and directly inhibiting bone-building cells while encouraging bone-resorbing cells. Other adipokines like visfatin and resistin activate key inflammatory pathways and drive the release of the same cytokines, including TNF-alpha and IL-6, that characterize RA inflammation.
Fat tissue stored within muscles can also release pro-inflammatory adipokines that contribute to the systemic inflammation seen in RA, adding another layer to the relationship between body composition and disease risk.
Epigenetic Changes: Where Genes Meet Environment
Epigenetics helps explain how environmental exposures actually translate into disease. Rather than changing your DNA sequence, factors like smoking, diet, and obesity alter how your genes are read. The three main mechanisms are chemical tags on DNA (methylation), modifications to the proteins that DNA wraps around (histones), and small RNA molecules that silence specific genes.
In RA, the synovial lining cells show widespread reduction in DNA methylation, which tends to activate genes that should be kept quiet. This happens in part because these cells are proliferating rapidly, using up the chemical building blocks needed for normal methylation. The result is that inflammatory genes and growth-promoting genes become inappropriately active. Women appear particularly vulnerable to this process, as high levels of certain metabolic byproducts associated with reduced methylation have been linked to increased RA risk specifically in women.
Smoking contributes epigenetically as well, altering the activity of a family of proteins involved in modifying histones. These changes can persist long after someone quits smoking, which may explain why former smokers retain elevated RA risk for years. The encouraging aspect of epigenetics is that these changes are, in principle, reversible. Some current RA treatments already work in part by restoring normal epigenetic patterns in immune cells, normalizing the function of regulatory T cells that help keep autoimmune responses in check.