Inflammatory arthritis happens when your immune system or metabolic processes drive inflammation in and around your joints, rather than simple wear and tear. Unlike osteoarthritis, which results from cartilage breaking down over time, inflammatory arthritis involves your body actively attacking joint tissue or depositing irritating crystals inside joints. The causes vary depending on the specific type, but they generally fall into a few well-understood categories: immune system malfunction, genetic susceptibility, environmental triggers, and metabolic imbalances.
The Immune System Turns on Your Joints
In the most common forms of inflammatory arthritis, including rheumatoid arthritis and psoriatic arthritis, the underlying problem is an immune system that has lost the ability to distinguish your own tissue from foreign invaders. This misdirected attack primarily targets the synovium, the thin membrane lining your joints. The synovium normally produces fluid that lubricates and nourishes the joint. When immune cells flood into it, the membrane becomes inflamed, thickened, and painful.
This process doesn’t start overnight. Rheumatoid arthritis is now understood to be the end result of a multi-year buildup in which the immune system gradually becomes dysregulated, likely beginning at mucosal surfaces like the lungs, mouth, or gut, before inflammation eventually localizes in the joints. During this buildup, the body may produce detectable antibodies years before any joint symptoms appear. Once the immune attack becomes established in the synovium, it triggers chronic inflammation that, without treatment, progressively damages cartilage and bone.
The immune cells involved release signaling molecules that amplify the inflammatory response. Three of the most important are TNF-alpha, IL-6, and IL-17. These molecules recruit more immune cells to the joint, stimulate the growth of destructive tissue, and break down cartilage and bone. They also circulate throughout the body, which is why inflammatory arthritis often causes fatigue, brain fog, and even depression in addition to joint pain.
Genetics Load the Gun
Your genes don’t cause inflammatory arthritis on their own, but certain genetic variants dramatically increase your susceptibility. For rheumatoid arthritis, a group of gene variants known as the “shared epitope” on the HLA-DRB1 gene is the strongest known genetic risk factor. People who carry this variant are more likely to develop a specific, often more aggressive form of the disease marked by particular antibodies in the blood.
For ankylosing spondylitis, a condition that primarily inflames the spine and sacroiliac joints, a gene variant called HLA-B27 is the major genetic player. Carrying HLA-B27 significantly raises the risk of developing the disease. But genetics is only part of the story: about 75 percent of children who inherit HLA-B27 from a parent with ankylosing spondylitis never develop the condition themselves. And some people develop ankylosing spondylitis without carrying the variant at all. Additional gene variants affecting immune regulation, including ones involved in how the body processes inflammatory signals, also contribute smaller amounts of risk.
This gap between genetic risk and actual disease is one of the clearest signs that inflammatory arthritis requires environmental or lifestyle triggers to develop, not just the right genes.
Smoking and the Gene-Environment Interaction
Smoking is the single best-studied environmental trigger for rheumatoid arthritis, and the risk it creates is not just additive. It multiplies. Men who have ever smoked face roughly double the risk of developing RA compared to men who never smoked. For the antibody-positive form of RA specifically, current male smokers face nearly four times the risk.
What makes smoking especially dangerous is how it interacts with genetics. In people who carry the shared epitope gene variant, smoking appears to trigger the production of antibodies against citrullinated proteins, a hallmark of aggressive rheumatoid arthritis. Researchers found that the odds of developing these antibodies were 5.3 times higher in people who both smoked and carried the shared epitope, compared to people with neither risk factor. That’s more than the two risks simply added together, suggesting a biological synergy: smoking in genetically susceptible people creates a perfect storm for autoimmune joint disease.
The likely mechanism involves the lungs. Smoking irritates and damages lung tissue, which can trigger a process called citrullination, where the body chemically modifies certain proteins. In susceptible people, the immune system then recognizes these modified proteins as foreign and begins producing antibodies against them. Those antibodies eventually find their way to the joints.
Bacteria in Your Gut and Mouth
The connection between the gut microbiome and inflammatory arthritis has become one of the most active areas of research in rheumatology. A specific gut bacterium called Prevotella copri has been found at elevated levels in people with early, untreated rheumatoid arthritis. Its abundance correlates with clinical markers of disease activity. Researchers have even detected shifts toward higher Prevotella levels in people who show preclinical signs of RA but haven’t yet developed joint symptoms, suggesting the gut changes may precede the disease rather than result from it.
The mouth harbors its own relevant pathogen. A bacterium called Porphyromonas gingivalis, the primary driver of periodontal gum disease, is the only known bacterium that can citrullinate proteins the same way human cells do. It produces its own version of the enzyme responsible for this chemical modification, and it works differently from the human version: it can function in low-calcium environments and targets the ends of proteins. In animal studies, infection with P. gingivalis significantly increased autoantibodies against both collagen and citrullinated proteins, but only when the bacterium retained its citrullination ability. A mutant strain without this enzyme didn’t produce the same autoimmune response. This finding provides a plausible biological route from gum disease to rheumatoid arthritis in genetically susceptible people.
Epigenetic Changes Amplify Inflammation
Even without altering your DNA sequence, environmental exposures can change how your genes are read. This happens through epigenetic modifications, chemical tags that attach to DNA and either silence or activate specific genes. In inflammatory arthritis, one critical change involves a process called DNA methylation, which adds small chemical groups to gene promoters and can shut them down.
During active arthritis, researchers have observed a dramatic increase in methylation across the genome of immune cells. One particularly important target is a gene that normally acts as a brake on the production of neutrophils, a type of white blood cell that drives tissue damage in arthritic joints. In animal models of arthritis, this gene’s promoter region was methylated at levels reaching 90 percent during peak disease, effectively silencing its protective function. Without this brake, the bone marrow overproduces inflammatory cells that flood the joints and accelerate destruction, including cells that break down bone itself.
Inflammatory signals like TNF-alpha, one of the same molecules driving joint inflammation, can themselves increase methylation of this protective gene by 25 to 40 percent. This creates a self-reinforcing cycle: inflammation silences genes that would normally restrain the immune response, which generates more inflammation.
Gout: A Metabolic Cause
Not all inflammatory arthritis stems from autoimmunity. Gout is caused by the buildup and crystallization of uric acid, a waste product from the breakdown of purines found in certain foods and produced naturally by the body. When blood levels of uric acid exceed 6.8 mg/dL, the saturation point, urate can begin forming needle-shaped crystals that deposit in joints and surrounding soft tissue.
The crystals themselves are intensely irritating to the immune system. When white blood cells encounter them, they mount a powerful inflammatory response that produces the classic gout flare: sudden, severe pain, redness, and swelling, most commonly in the big toe but potentially in any joint. Interestingly, flares are typically triggered not by a sustained high level of uric acid but by acute changes in the level, either a rapid rise or a sudden drop. This is why a gout attack can sometimes follow starting a medication that lowers uric acid, or after a night of heavy drinking that spikes it.
The underlying problem in most gout cases is insufficient excretion of uric acid by the kidneys rather than overproduction. Diet, alcohol, obesity, kidney function, and certain medications all influence uric acid levels and flare risk.
Other Contributing Factors
Sex hormones play a significant role. Rheumatoid arthritis is two to three times more common in women than men, and it often appears during hormonal transitions like the postpartum period or perimenopause. This suggests that estrogen and other hormones influence immune regulation in ways that affect susceptibility.
Obesity contributes to inflammatory arthritis risk beyond just mechanical stress on joints. Fat tissue actively produces inflammatory molecules, creating a low-grade systemic inflammation that can prime the immune system for an autoimmune response. In gout, higher body weight is directly associated with higher uric acid levels.
Infections beyond those in the gut and mouth can also trigger reactive arthritis, a form of inflammatory arthritis that develops after bacterial infections of the urinary tract, gut, or genitals. This type typically affects the knees, ankles, and feet and often resolves within months, though it can become chronic in some people, particularly those carrying the HLA-B27 gene variant.