Behçet’s disease has no single cause. It develops from a combination of genetic susceptibility, immune system dysfunction, and environmental triggers that together produce widespread inflammation of blood vessels throughout the body. The disease sits at an unusual crossroads between autoimmune and autoinflammatory conditions, which is part of why pinpointing a cause has been so difficult.
Genetic Risk: The HLA-B51 Connection
The strongest genetic link to Behçet’s disease is a gene variant called HLA-B51, which plays a role in how the immune system identifies threats. A meta-analysis of nearly 5,000 patients and over 16,000 healthy controls estimated that this single gene variant accounts for 32 to 52 percent of the population-level risk for developing the disease. If you carry HLA-B51 and live in a region where Behçet’s is common (Turkey, the Middle East, East Asia), your odds of developing the disease are 5 to 10 times higher than someone without it. In North America, that risk drops to about 2.35 times higher.
HLA-B51 doesn’t act alone. It interacts with another gene called ERAP1, which helps process proteins inside cells for immune recognition. When certain variants of both genes are present together, they amplify each other’s effect. Genome-wide studies have also identified variants in genes involved in immune signaling, particularly those controlling inflammatory messenger molecules. These additional genetic factors help explain why not everyone with HLA-B51 develops Behçet’s, and why some people without it still do.
An Immune System That Overreacts
Behçet’s disease is fundamentally a problem of immune regulation. The body’s first-responder white blood cells, called neutrophils, become hyperactivated. These overactive neutrophils cause damage through two main pathways.
First, they produce excessive amounts of reactive oxygen species, essentially corrosive molecules that damage the inner lining of blood vessels. This damage allows inflammatory cells to cross the vessel wall into surrounding tissue, causing injury. These oxygen species also alter the structure of clotting proteins in the blood, making clots harder for the body to break down. That’s one reason vascular complications, including blood clots, are so common in Behçet’s.
Second, neutrophils release web-like structures made of DNA and antimicrobial proteins. These structures are normally used to trap bacteria, but in Behçet’s they accumulate and trigger a chain reaction: immune cells called macrophages try to clean them up, get activated in the process, and release a sustained flood of inflammatory signals. This creates a self-perpetuating cycle of inflammation that can affect virtually any organ system.
Beyond neutrophils, specialized immune cells called Th1 and Th17 cells drive much of the chronic inflammation. These cells produce a cocktail of inflammatory signaling molecules, including TNF-alpha and IL-17, that keep the immune response elevated. Normally, the body has regulatory cells that dial inflammation back down. In Behçet’s patients, an overactive Th1 response appears to impair this regulatory brake, letting inflammation persist unchecked.
Environmental Triggers
Genetics loads the gun, but something in the environment pulls the trigger. Two infectious agents have been consistently linked to Behçet’s flares and onset: a common oral bacterium called Streptococcus sanguinis and herpes simplex virus. The leading theory is molecular mimicry. Proteins on these microbes resemble proteins found naturally in human tissue, particularly heat-shock proteins. The immune system mounts a response against the microbe but then mistakenly attacks the body’s own cells that share similar-looking protein structures. In the eye, for instance, immune complexes deposit in the tissue lining the iris and trigger an inflammatory cascade that can threaten vision.
Gut bacteria also appear to play a role. Compared to healthy people living in the same household and eating the same diet, Behçet’s patients have lower gut microbial diversity and are significantly depleted in bacteria that produce butyrate, a short-chain fatty acid critical for maintaining the intestinal lining and regulating immune responses. Two bacterial groups in particular, Roseburia and Subdoligranulum, were found at roughly half the levels seen in healthy housemates. This reduction in butyrate production may weaken the gut’s ability to keep inflammation in check, potentially contributing to the systemic immune dysfunction seen in the disease.
Why Geography Matters
Behçet’s disease clusters dramatically along the ancient Silk Road trading route. Prevalence in Turkey ranges from 70 to 420 cases per 100,000 people. In the United States and Europe, it drops to 0.1 to 7.5 per 100,000. This geographic pattern reflects both the distribution of HLA-B51 in these populations and likely environmental factors that remain poorly understood.
Interestingly, the disease appears to be declining in some affluent East Asian countries. In South Korea, incidence fell from 7.47 per 100,000 in 2006 to 2.51 per 100,000 in 2015. Japan has seen similar trends, with the proportion of Behçet’s among hospital uveitis patients dropping from 23% in the early 1980s to about 6% by 2002. Researchers have speculated that improvements in oral health and hygiene may be reducing exposure to the bacterial triggers that initiate the disease in genetically susceptible people.
Not Quite Autoimmune, Not Quite Autoinflammatory
Behçet’s disease doesn’t fit neatly into either of the two major categories of immune disorders. It shares features with autoinflammatory diseases: neutrophil hyperactivation, elevated levels of certain inflammatory molecules, a relapsing-remitting pattern of flares, and the absence of any specific autoantibody. It even shares genetic overlap and geographic distribution with familial Mediterranean fever, a classic autoinflammatory condition. Both respond to colchicine, an anti-inflammatory medication.
Yet Behçet’s also has hallmarks of autoimmune disease. Its strong association with HLA-B51 mirrors the genetic patterns seen in autoimmune conditions. Certain symptoms respond to drugs that suppress T cells, and the Th17 pathway, a key driver of autoimmune disease, plays a significant role in its inflammation. This dual nature means the disease likely involves both an innate immune system that overreacts to triggers and an adaptive immune system that sustains the damage once started.
How Vascular Damage Develops
Up to 40% of Behçet’s patients develop vascular complications, making it one of the few forms of vasculitis that affects both arteries and veins. Young men are disproportionately affected. The combination of vessel wall inflammation from neutrophil-driven damage and the altered clotting environment created by reactive oxygen species and DNA-based neutrophil traps creates conditions ripe for blood clots and vessel wall weakening. This vascular involvement is one of the leading causes of serious complications and death in Behçet’s disease, which is why the underlying inflammatory process, rather than any single symptom, defines the severity of the condition.