Lupus is a disease in which the immune system attacks the body’s own tissues, causing inflammation that can damage virtually any organ. The effects range from painful joints and skin rashes to life-threatening kidney failure and heart disease. About one-third to one-half of people with lupus develop neurological or psychiatric symptoms, and up to half develop clinically evident kidney disease. Understanding which systems are at risk helps explain why lupus looks so different from one person to the next.
An estimated 204,000 people in the United States have systemic lupus erythematosus (SLE), the most common and serious form. Nine out of ten are women, with the highest risk among women of childbearing age (15 to 44). Black and American Indian/Alaska Native women are two to three times more likely than white women to develop the disease.
What Goes Wrong in the Immune System
In a healthy body, the immune system clears away dead cells and fights infections. In lupus, the cleanup process breaks down. The body fails to properly clear cellular debris, especially fragments of DNA and RNA. The immune system mistakes these leftover nucleic acids for foreign invaders and produces autoantibodies that bind to them, forming clumps called immune complexes. These complexes settle into tissues throughout the body and trigger waves of inflammation.
Two key drivers keep this cycle going. First, the body ramps up production of a signaling molecule called type I interferon, which acts like a persistent alarm telling immune cells to stay activated. This interferon signature is present in 60 to 85 percent of lupus patients. Second, specialized immune cells (T follicular helper cells and T peripheral helper cells) push B cells to churn out more autoantibodies, fueling a self-reinforcing loop of immune activation and tissue damage. Some people also have fewer copies of genes responsible for complement proteins, which are supposed to clear immune complexes from the bloodstream. When that clearance system is weak, more complexes accumulate in organs.
Joints and Muscles
Joint pain is often the first symptom people notice. Lupus arthritis typically affects the same joints on both sides of the body, especially the small joints of the hands, wrists, and feet. Larger joints like knees, shoulders, and elbows can also be involved. Most people experience polyarthritis or polyarthralgia, meaning five or more joints are affected at once, with morning stiffness lasting at least 30 minutes.
The pattern closely resembles rheumatoid arthritis, but there’s an important distinction: lupus arthritis rarely erodes bone. Over time, however, looseness in tendons and ligaments can change the shape of the hands, creating visible deformity even without the structural bone damage seen in rheumatoid arthritis. Muscle aches and weakness are also common during flares.
Skin and Sun Sensitivity
The butterfly rash, also called a malar rash, is one of lupus’s most recognizable signs. It spreads across both cheeks and the bridge of the nose in a shape that resembles butterfly wings, typically sparing the laugh lines that run from the nose to the corners of the mouth. On lighter skin, the rash appears red or pink. On darker skin, it can look brown, black, or purple. It may be flat, raised, or scaly, and it can burn, itch, or feel painful.
Ultraviolet light is a major trigger. Immune cells in the skin react to UV exposure by releasing inflammatory chemicals, a process called photosensitivity. For many people with lupus, even brief sun exposure can set off not just skin rashes but full-body flares. Other skin-related problems include nonscarring hair loss and oral ulcers, both of which are common enough to be included in the formal classification criteria for the disease.
Kidney Damage
The kidneys are one of the organs most vulnerable to lupus. Clinically evident kidney disease develops in up to half of all SLE patients, and the risk is not evenly distributed. Black patients face a 34 to 51 percent chance, Hispanic patients 31 to 43 percent, and Asian patients 33 to 55 percent, compared with 14 to 23 percent for white patients.
Lupus nephritis occurs when autoantibody-laden immune complexes deposit in the kidney’s filtering units (glomeruli), triggering complement activation and drawing in inflammatory cells like neutrophils. The result is progressive damage to the kidney’s ability to filter blood. Early signs include protein in the urine, blood in the urine, and rising creatinine levels. About 30 percent of all SLE patients eventually develop elevated creatinine, a marker of declining kidney function. Because no blood or urine test can fully characterize the severity of kidney involvement, a kidney biopsy is the standard method for classifying how advanced the damage is and guiding treatment decisions.
Heart and Blood Vessels
Lupus raises the risk of heart disease in several distinct ways. The most common cardiac complication is pericarditis, inflammation of the sac surrounding the heart, which causes sharp chest pain that worsens with deep breaths. Lupus can also inflame the heart muscle itself (causing fatigue, chest pain, and irregular heartbeat) or the inner lining and valves of the heart, where it can produce small clumps of material that break off and form blood clots.
Beyond direct inflammation, lupus accelerates coronary artery disease. Chronic inflammation damages blood vessel walls and promotes plaque buildup, narrowing the arteries that supply the heart. High blood pressure is common and compounds the risk to both the heart and kidneys. Lupus can also cause vasculitis, inflammation in blood vessel walls that restricts blood flow to organs.
Some people with lupus develop antiphospholipid antibody syndrome, a condition that dramatically increases the risk of blood clots inside vessels. These clots can cause stroke, heart attack, or pulmonary embolism, a blockage in the lungs that can be life-threatening.
Lungs
The most common lung problem is pleuritis, inflammation of the thin tissue covering the lungs. When these inflamed surfaces rub together during breathing, it causes chest pain and shortness of breath. Fluid can also accumulate between the lung and chest wall (pleural effusion), compressing lung tissue. Less commonly, lupus causes pneumonitis, inflammation within the lung tissue itself, or pulmonary hypertension, high blood pressure in the lung’s blood vessels. Pulmonary hypertension is rare but serious, as it strains the right side of the heart over time.
Brain and Nervous System
Roughly one-third to one-half of people with lupus experience neurological or psychiatric symptoms at some point. The range is broad. Cognitive impairment, sometimes called “lupus fog,” involves difficulty with memory, concentration, and word-finding. Depression and anxiety are common. Severe headaches occur frequently during flares.
More serious neuropsychiatric manifestations include seizures, psychosis (with hallucinations or disordered thinking), and stroke. Lupus can also cause optic neuritis (inflammation of the nerve behind the eye), myelitis (inflammation of the spinal cord), and aseptic meningitis. These complications can result from autoantibodies directly affecting nerve tissue, from blood clots blocking circulation to the brain, or from inflammation of small blood vessels in the central nervous system.
Blood Cells
Lupus frequently disrupts the production and survival of blood cells. Anemia is the most common blood abnormality, affecting roughly 56 percent of patients with hematologic involvement. In some cases, the immune system directly destroys red blood cells, a process called autoimmune hemolytic anemia, which occurs in about 13 percent of these patients.
Low white blood cell counts (leukopenia) affect about 12 percent, leaving people more susceptible to infections. Low platelet counts (thrombocytopenia) affect about 9 percent, increasing the risk of bleeding and bruising. These blood changes can be among the earliest laboratory clues that something is wrong, sometimes appearing before joint pain or rash.
What Triggers Flares
Lupus tends to cycle between periods of relative calm and active flares. Ultraviolet light is one of the most reliable triggers, but it is far from the only one. Infections can ramp up the immune system in ways that spill over into autoimmune activity. Emotional stress and physical exhaustion are commonly reported triggers, likely because stress hormones modulate immune function.
Certain environmental exposures have also been linked to lupus flares or even disease onset. Hydrazines, chemicals found in tobacco smoke and various industrial products, have been associated with drug-induced lupus in some studies. Mercury and other heavy metals may activate autoimmune processes. Estrogen plays a role as well, which helps explain why the disease overwhelmingly affects women of reproductive age and why flares can worsen during pregnancy or around menstrual cycles.
How Lupus Is Classified
Diagnosing lupus is notoriously difficult because its symptoms overlap with so many other conditions. The current classification system, developed jointly by the European and American rheumatology organizations, uses a point-based approach. A positive antinuclear antibody (ANA) test at a specific threshold is required as an entry criterion, and then clinical and immunologic features are each assigned point values.
The system reflects how seriously different manifestations affect the body. Joint involvement and the butterfly rash each score 6 points. Seizures score 5. The most severe form of lupus nephritis, confirmed by kidney biopsy, scores the maximum of 10. On the immunologic side, specific antibodies (anti-double-stranded DNA and anti-Smith) each contribute 6 points, while low levels of complement proteins add 3 to 4. A total score of 10 or more, with at least one clinical criterion, leads to a classification of SLE. This scoring system helps explain why two people with lupus can look so different: the disease can reach that threshold through many different combinations of organ involvement.