Arthritis develops when the protective structures inside your joints break down, get attacked by your own immune system, or become damaged by crystal deposits. About 21% of American adults have been diagnosed with some form of arthritis, and the reasons behind it vary widely depending on the type. Understanding what’s actually happening inside the joint helps explain why some people develop it and others don’t.
How Joints Break Down in Osteoarthritis
Osteoarthritis, the most common form, happens when the cartilage cushioning your joints wears away faster than your body can repair it. Healthy cartilage is maintained by a constant balance between building new tissue and clearing out old tissue. When something disrupts that balance, whether it’s aging, an injury, or years of heavy use, the scales tip toward destruction.
The breakdown is driven by enzymes that chew through the two main components of cartilage: collagen (the structural fibers that give cartilage its strength) and aggrecan (the molecule that helps cartilage absorb shock). Mechanical stress on the joint triggers cells in the cartilage to release these destructive enzymes, along with inflammatory signals that accelerate the damage. Once enough cartilage is lost, bone grinds against bone, causing the pain, stiffness, and swelling most people associate with arthritis.
This process can take decades or just a few years, depending on how many risk factors are stacked against you.
Why the Immune System Attacks Your Joints
Rheumatoid arthritis works through an entirely different mechanism. Instead of wear-and-tear, your immune system mistakenly identifies the tissue lining your joints (the synovium) as a threat and launches a sustained attack against it.
In a healthy joint, the synovial lining is thin, just a few cells thick. In rheumatoid arthritis, it becomes heavily infiltrated with immune cells: T cells, B cells, macrophages, and mast cells. These cells flood the joint with inflammatory signals that do three things simultaneously. They destroy cartilage, they activate specialized bone-dissolving cells that erode the bone itself, and they recruit even more immune cells to the area, creating a self-reinforcing cycle of inflammation.
B cells play a particularly important role. They produce antibodies directed against the body’s own tissues, secrete inflammatory chemicals, and present fragments of the body’s own proteins to other immune cells, essentially training the immune system to keep attacking. The effectiveness of treatments that eliminate B cells from the bloodstream confirmed just how central they are to the disease.
Genetics and the Shared Epitope
Your genes don’t guarantee you’ll get arthritis, but certain genetic variations significantly raise your odds. For rheumatoid arthritis, the most important known genetic risk factor is a gene called HLA-DRB1. Two specific versions of this gene are strongly linked to developing the disease.
These gene variants produce a slightly altered version of an immune protein that normally helps identify foreign invaders like bacteria and viruses. The altered protein contains a specific sequence known as the “shared epitope,” which makes it more likely to bind to the body’s own proteins instead of foreign ones. When that happens, the immune system treats normal joint tissue as something that needs to be destroyed.
Having these gene variants doesn’t mean you’ll definitely develop rheumatoid arthritis. Environmental triggers still need to pull the trigger. But it does mean your immune system is, in a sense, primed to misfire.
What Your Gut Bacteria Have to Do With It
One of the more surprising findings in recent years involves the gut microbiome. People who are at risk for or newly diagnosed with rheumatoid arthritis consistently show an overgrowth of certain gut bacteria, particularly strains from the Prevotella family. Mouse studies have shown that transplanting these bacterial strains from patients with rheumatoid arthritis can actually trigger the disease in animals.
The leading theory is that inflammation starts at mucosal surfaces like the gut lining, where the immune system first encounters these bacteria. That localized inflammation activates immune cells, which then begin producing antibodies. Over time, this mucosal immune response spreads and becomes systemic, eventually reaching the joints. Longitudinal research found that people who went on to develop rheumatoid arthritis showed gut microbiome instability roughly 10 months before their symptoms appeared, a shift that didn’t happen in people who stayed healthy.
How Extra Weight Multiplies Joint Stress
Every pound of body weight translates into far more than a pound of force on your joints. Walking on flat ground puts force equal to 1.5 times your body weight on each knee. Going up or down stairs multiplies that to two to three times your body weight. Squatting to pick something up off the floor loads each knee with four to five times your body weight.
This means that even 10 extra pounds adds 30 to 50 pounds of pressure on your knees during everyday activities. Over years, that extra mechanical load accelerates cartilage breakdown. But weight also contributes to arthritis through a less obvious pathway: fat tissue produces inflammatory chemicals that circulate throughout the body and damage joint tissue directly, which is why obesity increases arthritis risk even in non-weight-bearing joints like the hands.
Gout: When Crystals Form Inside the Joint
Gout is a form of arthritis caused by a buildup of uric acid in the blood. Uric acid is a normal waste product from breaking down certain foods and your body’s own cells. When blood levels exceed roughly 6 mg/dL, uric acid can crystallize into sharp, needle-like deposits inside joints, most commonly the big toe.
These crystals trigger an intense inflammatory response. The joint becomes red, swollen, hot, and extraordinarily painful, often overnight. Gout attacks can resolve on their own, but repeated flares cause progressive joint damage over time. High-purine diets (red meat, organ meats, certain seafood), alcohol, kidney problems, and genetics all influence how efficiently your body clears uric acid.
How Injuries Set the Stage Years Later
A joint injury doesn’t just hurt at the time. Between 20% and 50% of people who experience significant joint trauma go on to develop osteoarthritis in that joint. Post-traumatic arthritis accounts for roughly 12% of all osteoarthritis cases.
The timeline varies enormously. Some people develop arthritis within a year of the injury. Others remain symptom-free for 10 to 20 years before the damaged joint starts deteriorating. Injuries like torn ligaments, meniscus tears, and fractures that extend into the joint surface change the mechanics of how force is distributed across the cartilage. Even after surgical repair, that altered loading pattern can gradually wear the cartilage down.
Jobs That Wear Joints Down
Certain occupations carry substantially higher arthritis risk because of the repetitive stress they place on joints. An Oxford University analysis found that floor layers, bricklayers, and carpenters have about 2.5 times the odds of developing knee osteoarthritis compared to people with desk jobs. Agricultural workers face up to 64% increased odds, and builders and construction workers see a 63% increase. Even unpaid housework, with its constant kneeling, lifting, and standing, raises risk by up to 93%.
The specific movements that drive the damage are excessive kneeling, squatting, prolonged standing, heavy lifting, and climbing stairs. These activities load the knee joint repeatedly in positions where cartilage compression is highest.
Why Women Are More Affected
Women account for 60% of osteoarthritis cases worldwide, and the gap widens significantly after age 40. Several factors contribute. Women tend to have different joint alignment and anatomy than men, with wider hips that change the angle of force through the knee. Differences in muscle strength around the joint mean less muscular support to absorb impact. Hormonal shifts, particularly the drop in estrogen around menopause, appear to accelerate cartilage loss and increase inflammation.
For rheumatoid arthritis, the gender gap is even more pronounced. Women develop it roughly two to three times more often than men, likely because of estrogen’s complex effects on the immune system: it can both stimulate and suppress immune activity depending on concentration, making the immune system less stable and more prone to the kind of misfiring that triggers autoimmune disease.