Multiple sclerosis (MS) is caused by the immune system mistakenly attacking myelin, the protective coating around nerve fibers in the brain and spinal cord. This assault damages the brain’s ability to send electrical signals efficiently, leading to the wide range of symptoms MS is known for. The process involves a chain of events: immune cells that normally fight infections become misdirected, cross into the brain, and destroy tissue they should leave alone. What triggers this misdirection appears to be a combination of genetic susceptibility, viral infection, and environmental factors that stack on top of each other.
How the Immune System Attacks the Brain
In a healthy brain, nerve fibers are wrapped in myelin, a fatty insulating layer produced by specialized cells called oligodendrocytes. Myelin allows electrical signals to travel quickly along nerves, similar to how rubber insulation protects a wire. In MS, immune cells that belong in the bloodstream cross into the brain and begin stripping this insulation away.
The primary attackers are T cells, a type of white blood cell. Two varieties do most of the damage. CD4+ T cells coordinate the immune response and recruit other inflammatory cells to the site. CD8+ T cells deliver the killing blow: after locking onto a target, they release a protein called perforin that punches holes in cell membranes, then inject enzymes that trigger cell death. This process destroys both the myelin sheath and, over time, the oligodendrocytes that produce it. B cells also play a role, producing antibodies found in the spinal fluid of most MS patients, a hallmark used in diagnosis.
The result is a demyelinating plaque, the signature lesion of MS visible on brain scans. These plaques form primarily in the brain’s white matter, where bundles of myelinated nerve fibers run between regions. White matter lesions are heavily infiltrated with immune cells. Gray matter lesions also occur but look different under a microscope: they contain far fewer immune cells, suggesting a partially different mechanism of damage, possibly driven more by the brain’s own resident immune cells and the local chemical environment than by cells invading from the blood.
How Immune Cells Get Past the Blood-Brain Barrier
The brain is normally sealed off from the rest of the body by the blood-brain barrier, a tightly constructed wall of cells lining blood vessels in the brain. This barrier is extremely selective about what it lets through. In MS, activated immune cells in the bloodstream release inflammatory chemicals, reactive oxygen species, and enzymes that loosen the barrier’s tight junctions. Once the barrier becomes leaky, T cells and B cells flood into brain tissue where they don’t belong.
This isn’t a one-time event. The state of immune activation in the blood directly correlates with barrier breakdown. Treatments that reduce peripheral immune cell activation also reduce the number of new active lesions visible on MRI, confirming that the barrier breach is driven by the immune system itself rather than being a passive structural failure.
The Epstein-Barr Virus Connection
Nearly all people with MS have been infected with Epstein-Barr virus (EBV), the virus that causes mononucleosis. A landmark study of over 10 million military personnel found that EBV infection preceded MS onset in virtually every case, making it the strongest known environmental risk factor. The leading explanation is a process called molecular mimicry: certain EBV proteins, particularly one called EBNA1, share a structural resemblance to myelin proteins in the brain.
When the immune system builds a response against EBNA1, some of those T cells become cross-reactive. They recognize both the viral protein and myelin basic protein, one of the key components of the myelin sheath. Researchers have isolated EBNA1-targeting T cells from MS patients and shown that these same cells attack myelin antigens and release inflammatory signals. In other words, the immune system’s memory of fighting EBV becomes a case of mistaken identity, with the brain’s own insulation as the collateral target.
Genetic Susceptibility
MS is not directly inherited, but genetics load the gun. The strongest known genetic risk factor is a variant in the HLA gene region called HLA-DRB1*15:01. Carrying this variant confers a 3-fold increased risk of developing MS. The HLA genes encode proteins that present fragments of foreign invaders to T cells, essentially showing the immune system what to attack. A variation in this system could make it more likely that myelin fragments get mistakenly flagged as threats.
Over 200 other genetic variants have been linked to MS risk, most of them involved in immune function. Each one contributes a small amount of risk on its own. Having a first-degree relative with MS raises your risk to roughly 2 to 4 percent, compared to about 0.1 percent in the general population. Identical twins share the disease only about 25 to 30 percent of the time, which confirms that genes alone are not enough. Something in the environment has to pull the trigger.
Vitamin D and the Latitude Effect
MS prevalence increases dramatically the farther you live from the equator. Between the equator and 60 degrees latitude (roughly the level of Anchorage or Stockholm), prevalence rises up to 10-fold. This gradient is established early in life: a New Zealand study found the latitude effect was strongest at birth and persisted until about age 12 before gradually weakening. Where you grow up matters more than where you live as an adult.
The most likely explanation is sunlight exposure and its effect on vitamin D production. A large study of seven million U.S. military personnel found that those with vitamin D blood levels above 40 ng/mL had a 62 percent lower chance of developing MS compared to those with the lowest levels. People in the lowest vitamin D group (below about 8 ng/mL) had roughly double the risk of those in the highest group. Vitamin D regulates immune function in ways that suppress the kind of autoimmune activity seen in MS, which may explain why insufficient levels during childhood create a window of vulnerability.
Smoking and Other Lifestyle Factors
Smoking increases the risk of developing MS by about 50 percent compared to never smoking. Among women, current smokers have a 60 percent higher incidence rate than those who have never smoked, while past smokers show a smaller, non-significant increase of about 20 percent. The effect size is modest compared to EBV or genetics, but smoking also accelerates disease progression after diagnosis. Cigarette smoke contains thousands of compounds that promote inflammation and oxidative stress, both of which can aggravate the immune dysfunction already present in MS.
The Gut Microbiome’s Role
People with MS consistently show an altered balance of gut bacteria. The pattern involves a decrease in bacteria that produce butyrate, a short-chain fatty acid that helps regulate the immune system and maintain the intestinal lining. Specifically, MS patients tend to have lower levels of several butyrate-producing bacteria, including Faecalibacterium, Roseburia, Coprococcus, and Prevotella. At the same time, they show increases in Akkermansia and Ruminococcus, bacteria that can degrade the gut’s protective mucus layer and potentially promote inflammation.
The gut houses roughly 70 percent of the body’s immune cells, and the bacterial environment there shapes how those cells develop and behave. A gut microbiome that underproduces butyrate and other anti-inflammatory compounds may allow immune cells to become more aggressive, contributing to the autoimmune cascade that eventually reaches the brain. This connection between the gut and the brain is a two-way street: the inflammation from MS itself likely alters gut bacteria, creating a feedback loop.
How Nerve Damage Becomes Permanent
Early in the disease, the brain can partially repair myelin damage. Oligodendrocytes can regenerate and lay down new myelin in a process called remyelination. This is why many people with relapsing-remitting MS recover function between flare-ups. But the repair system has limits.
Repeated attacks eventually overwhelm the oligodendrocytes. These cells are metabolically demanding: they require enormous amounts of energy to produce and maintain myelin across thousands of nerve fibers. Their internal energy factories, the mitochondria, become dysfunctional during chronic MS. When mitochondria fail, oligodendrocytes can no longer synthesize the fatty compounds that make up myelin. They also become vulnerable to a form of cell death driven by iron buildup, where excess iron generates toxic reactions inside the cell. Once enough oligodendrocytes die and the axons they support are left permanently bare, nerve fibers themselves begin to degenerate. This transition from reversible myelin damage to irreversible nerve loss is what drives the shift from relapsing-remitting MS to progressive MS, where disability accumulates steadily.