Rheumatoid arthritis is caused by an immune system malfunction in which the body attacks its own joint tissues, but the chain of events that triggers this malfunction involves a complex interplay of genetics, environmental exposures, hormonal shifts, and even specific bacteria. Around 18 million people worldwide live with the condition, and about 70% of them are women. No single cause explains every case, but researchers have identified several converging factors that, together, paint a detailed picture of how and why RA develops.
The Core Problem: Your Immune System Targets Your Own Proteins
RA begins with a case of mistaken identity at the molecular level. Your body naturally modifies certain proteins through a process called citrullination, where one amino acid in a protein chain gets chemically converted into a slightly different one. This happens in healthy people all the time and is essential for normal skin formation and other functions. In people who develop RA, the immune system starts treating these modified proteins as foreign invaders and produces antibodies against them, known as anti-citrullinated protein antibodies (ACPAs).
These antibodies are remarkably specific to RA, showing up in roughly two-thirds of patients and appearing with over 98% specificity for the disease. What makes this especially striking is the timeline: ACPAs can be detected in blood up to 10 to 15 years before any joint pain or swelling appears. This means the immune system is already gearing up for an attack long before a person feels anything wrong. During this silent “pre-clinical” phase, inflammatory markers, immune cell activity, and autoantibody levels gradually climb until they finally spill over into visible disease.
Once the immune response reaches the joints, it inflames the synovial lining, the thin membrane that cushions and lubricates your joints. The cells in this lining begin behaving abnormally, multiplying aggressively and producing inflammatory chemicals that erode cartilage and bone over time.
Genetic Risk: The Shared Epitope
Certain gene variants dramatically increase susceptibility to RA. The most important ones sit in the HLA-DRB1 gene, which provides instructions for immune system proteins that help distinguish the body’s own cells from foreign ones. Two specific versions of this gene, HLA-DRB1*04 and HLA-DRB1*01, are strongly linked to higher RA risk. They share a common feature called the “shared epitope,” a particular sequence of amino acids that changes how the immune protein binds to other molecules.
Normally, these immune proteins grab fragments of viruses or bacteria and present them to other immune cells, triggering a targeted defense. The shared epitope alters this binding site in a way that makes the protein more likely to latch onto the body’s own citrullinated proteins instead, essentially misdirecting the immune response toward self-attack. People carrying these gene variants who also smoke or have other environmental exposures face a compounded risk, because the genetic and environmental factors reinforce each other.
Smoking: The Strongest Environmental Trigger
Cigarette smoking is the single most well-established environmental risk factor for RA. Light smokers (1 to 10 pack-years of exposure) have a 26% increased risk of developing the disease, while heavier smokers (more than 20 pack-years) face a 94% increased risk. The biological explanation ties directly back to citrullination: cigarette smoke triggers the citrullination of proteins in lung tissue, creating exactly the kind of modified proteins the immune system may then learn to attack.
Smoking also ramps up the number of immune cells in the lungs, increases the production of inflammatory signaling molecules like TNF-alpha, and damages connective tissue through the release of destructive enzymes. Both the intensity and duration of smoking correlate with higher levels of these inflammatory markers. The effect isn’t limited to the lungs, either. Smoking promotes gum disease, which introduces another pathway to RA through oral bacteria (more on that below).
Gum Disease and Oral Bacteria
One of the more surprising discoveries in RA research is the strong link to periodontal disease. People with gum disease lasting more than five years have a 69% higher risk of developing RA compared to those with healthy gums. The connection centers on a specific bacterium called Porphyromonas gingivalis, a major driver of aggressive gum disease.
This bacterium produces its own citrullinating enzyme, meaning it can modify human proteins in the same way that triggers the autoimmune response seen in RA. It also produces enzymes that chop up human proteins like fibrinogen and alpha-enolase into fragments, which its citrullinating enzyme then modifies further, generating dozens of new citrullinated protein fragments. In animal studies, oral exposure to this bacterium significantly raised ACPA levels in the blood, and citrullinated proteins along with bacterial components were found in ankle joints. When researchers used a version of the bacterium with the citrullinating enzyme removed, the autoimmune response didn’t develop, confirming the enzyme’s central role.
DNA from this bacterium has been found in the synovial fluid and tissue of RA patients at significantly higher rates than in people with other forms of joint inflammation (33% versus about 6%). Other oral bacteria also contribute. One species produces a toxin that forces immune cells called neutrophils to rupture, spilling their citrullinated proteins into surrounding tissue and potentially fueling the autoimmune cycle.
Gut Bacteria and the Microbiome
The mouth isn’t the only microbial environment that matters. Research published in the Annals of the Rheumatic Diseases found that people at risk for RA have a distinct gut bacterial signature. A specific strain of Prevotella copri is enriched in people at risk compared to healthy controls, and certain strains become more abundant as people progress toward clinical disease. At the same time, beneficial bacteria from the Lachnospiraceae family show consistent depletion in people who go on to develop RA.
The picture is nuanced. Not all Prevotella strains behave the same way. Some are enriched in people who progress to RA while others are actually depleted, suggesting that strain-level differences matter more than the presence of the bacterial family as a whole. Other related species, including Paraprevotella clara and Prevotella stercorea, also showed associations with RA clinical markers.
Why Women Are Affected More Often
Women develop RA two to three times more often than men, and hormonal shifts play a clear role. The typical onset occurs in adults in their sixties, and 55% of people living with RA are older than 55. Early menopause is associated with increased risk, likely because the abrupt drop in estradiol and progesterone promotes a pro-inflammatory environment by boosting levels of key inflammatory signaling molecules.
After menopause, the body’s primary estrogen shifts from the more potent estradiol to the weaker estrone, which may further sustain chronic inflammation and amplify autoimmune responses. Menopause has been linked not only to a higher likelihood of developing RA but also to faster joint damage progression and quicker functional decline in those who already have it. Interestingly, menopausal hormone therapy doesn’t appear to be protective. A meta-analysis found that current hormone therapy use is associated with a 15 to 18% higher RA risk, with long-term use of four years or more raising risk by about 19%.
Occupational and Industrial Exposures
Workplace exposures add another layer of risk, particularly for men. Silica dust exposure, common in construction, rock drilling, stone crushing, and masonry, more than doubles the risk of developing RA in men. Those in the most heavily exposed occupations, such as rock drilling and stone crushing, face a threefold increase in risk. The average duration of silica exposure among affected workers was around 20 years, and about two-thirds of exposed cases worked in building industry occupations.
RA prevalence is higher in industrialized countries overall, which may partly reflect greater exposure to environmental pollutants and occupational hazards alongside demographic factors like higher average age.
Epigenetic Changes: How Environment Alters Gene Behavior
Epigenetics helps explain how environmental exposures like smoking and pollution translate into lasting disease. In RA, the cells lining the joints show widespread changes in how their genes are regulated, specifically through a process called DNA methylation, which acts like a dimmer switch on gene activity. In RA patients, the DNA in synovial cells is extensively “hypomethylated,” meaning many genes that should be dialed down are instead turned up.
This altered gene activity affects how joint-lining cells grow, stick together, migrate, and interact with surrounding tissue, all of which contributes to the chronic inflammation and joint destruction seen in RA. Specific inflammatory genes, including those for key signaling molecules like IL-6 and IL-10, show increased activity as methylation on their control regions decreases over the course of the disease. These epigenetic changes help explain why RA joint cells behave so differently from normal ones, maintaining an aggressive, inflammation-promoting state even when the original trigger may no longer be present.