Multiple sclerosis doesn’t weaken your immune system. It’s the opposite: MS is a disease of an immune system that’s too active, specifically one that has lost the ability to tell the difference between foreign invaders and your own body. An estimated 2.8 million people worldwide live with MS, including nearly 914,000 in the United States, and in every case, the core problem is an immune system attacking healthy nerve tissue.
How MS Redirects the Immune System
In a healthy body, immune cells patrol for threats like bacteria, viruses, and abnormal cells. In MS, certain white blood cells lose their ability to distinguish threats from the body’s own tissue. They begin targeting myelin, the protective coating around nerve fibers in the brain and spinal cord. Myelin works like insulation on electrical wiring. When it’s damaged, nerve signals slow down, misfire, or stop entirely.
The process starts when a specific type of immune cell crosses into the brain and encounters myelin proteins. These cells treat the myelin as if it were a dangerous pathogen and trigger a full inflammatory response. They release signaling chemicals that recruit more immune cells to the site, amplifying the damage. The result is areas of inflammation and scarring (called lesions or plaques) scattered across the brain and spinal cord, which cause the wide range of symptoms MS is known for: numbness, vision problems, fatigue, difficulty walking, and cognitive changes.
A second branch of the immune system piles on. Certain immune cells produce antibodies that bind directly to the myelin surface or to the cells that manufacture myelin. Once these antibodies latch on, other immune cells and inflammatory proteins arrive to strip the myelin away. This antibody-driven destruction is one of the key mechanisms that turns a single episode of symptoms into a chronic, relapsing disease.
Overactive, Not Underactive
One common misconception is that having MS means your immune system is compromised. The disease itself does not make you immunodeficient. Your body’s ability to fight off colds, flu, and other infections remains fundamentally intact. The problem is misdirected aggression, not weakness.
In people without MS, the immune system has built-in brakes. Specialized regulatory cells keep inflammatory responses in check and prevent attacks on the body’s own tissues. In MS, that balance is disrupted. The inflammatory cells that drive damage become overrepresented, while the regulatory cells that would normally suppress them are less effective. This isn’t a system that’s broken. It’s a system that’s lost its ability to self-regulate.
The Epstein-Barr Virus Connection
Nearly every person with MS has been infected with Epstein-Barr virus (EBV), the virus responsible for mono. While 86 to 95 percent of the general population carries EBV antibodies, the rate in people with MS is essentially 100 percent. The odds of developing MS are roughly 13 times higher in people who’ve been infected with EBV compared to those who haven’t.
The leading explanation involves a phenomenon called molecular mimicry. Parts of the Epstein-Barr virus look structurally similar to proteins found in myelin. When the immune system builds a response against the virus, some of those immune cells may also react to myelin, essentially confusing the two. In genetically susceptible people, this cross-reactivity may be the spark that starts the autoimmune process. EBV also has the ability to reprogram certain immune cells it infects, potentially making them more likely to drive chronic inflammation.
Your Gut Plays a Role Too
The trillions of bacteria living in your digestive tract have a surprising amount of influence over immune function, and the gut microbiome in people with MS looks different from that of people without the disease. Certain gut bacteria produce metabolites that travel through the bloodstream and influence immune cell behavior throughout the body, including in the brain.
Some bacteria promote the growth of inflammatory immune cells that migrate to the central nervous system during active disease. Others do the opposite. One well-studied species produces a compound that transforms aggressive immune cells into regulatory ones, which then release anti-inflammatory signals and suppress the very cell types that drive MS damage. In one study, giving a short-chain fatty acid called propionate to newly diagnosed MS patients increased their regulatory immune cells, decreased their inflammatory cells, and reduced their annual relapse rate. The composition of your gut bacteria may be one of several environmental factors that tip the immune balance toward or away from disease activity.
How MS Treatments Change Immune Function
This is where the distinction between MS itself and MS treatment becomes important. While the disease doesn’t weaken your immune system, many of the medications used to manage it deliberately dial immune activity down. These treatments fall into a few broad categories, and they affect your defenses in different ways.
Some medications work by rebalancing the immune response rather than suppressing it. They boost anti-inflammatory signals and reduce pro-inflammatory ones, or they teach the immune system to tolerate myelin rather than attack it. These drugs generally don’t leave you significantly more vulnerable to infections. People taking these medications respond to vaccines about as well as people not on any treatment. Studies show that flu, pneumococcal, tetanus, and COVID-19 vaccines all produce strong antibody responses in patients on these types of therapies.
Other medications take a more aggressive approach. Some block immune cells from entering the brain entirely, which prevents new attacks but doesn’t broadly suppress immune function elsewhere. Others trap immune cells in the lymph nodes so fewer are circulating in the body. And a third group works by directly depleting certain immune cell populations. These more powerful therapies can genuinely reduce your ability to fight infections and respond to vaccines. In studies of COVID-19 vaccination, patients on cell-depleting therapies had antibody response rates around 40 percent, compared to 100 percent for patients on milder medications or no treatment at all.
The practical takeaway: if you’re on MS treatment, your infection risk and vaccine response depend heavily on which specific medication you’re taking. Two people with MS can have very different immune profiles based on their treatment plan alone.
Other Autoimmune Conditions Alongside MS
Because MS reflects a broader tendency toward immune dysregulation, people with the disease are more likely to develop other autoimmune conditions. About 4.5 percent of MS patients have at least one additional autoimmune disorder. The most common is Hashimoto’s thyroiditis, an autoimmune condition affecting the thyroid, found in roughly 2.5 percent of MS patients. Rheumatoid arthritis, lupus, psoriasis, Graves’ disease, and type 1 diabetes also appear at elevated rates.
These overlapping conditions share genetic risk factors and immune pathways. Researchers have found that certain gene variants involved in immune cell signaling are more common in people with both MS and lupus, for example. The same underlying immune imbalance that allows the body to attack myelin can, in some people, also lead to attacks on the thyroid, joints, or skin. Having MS doesn’t mean you’ll develop another autoimmune disease, but it does mean the underlying wiring of your immune system is predisposed to this kind of misfiring.