ALS is not classified as an autoimmune disease. It is a neurodegenerative disease, meaning motor neurons progressively break down and die rather than being attacked by a misdirected immune system. But the line between these two categories is blurrier than it used to be. A growing body of research shows that the immune system plays a significant, active role in ALS progression, and a landmark 2025 study found that many ALS patients mount an immune response against one of the disease’s own key proteins.
How ALS Is Officially Classified
ALS is the most common form of motor neuron disease. It destroys both the upper motor neurons in the brain’s motor cortex and the lower motor neurons in the spinal cord and brainstem, leading to progressive muscle weakness, paralysis, and eventually death. The hallmark features visible under a microscope are loss of motor neurons, shrinkage of the nerve tracts connecting the brain to the spinal cord, and the buildup of a misfolded protein called TDP-43 inside affected cells.
In a true autoimmune disease like multiple sclerosis, the immune system directly attacks healthy tissue as its primary action. In MS, immune cells strip the insulating coating off nerve fibers, and the nerve damage follows from that. ALS works the other way around: motor neurons degenerate first, and inflammation follows. MS has traditionally been viewed as an inflammatory disease with a secondary degenerative component. ALS has been considered a degenerative disease with a secondary inflammatory component. That distinction is what keeps ALS in the neurodegenerative category, even as the inflammatory piece turns out to be more important than scientists once thought.
Why the Immune System Still Matters in ALS
Microglia, the brain’s resident immune cells, are now recognized as key contributors to both the onset and progression of ALS. In healthy brains, microglia clear debris and support neurons. In ALS, they shift into an inflammatory state, pumping out signaling molecules that damage surrounding tissue. They also release reactive oxygen species and fail to clean up toxic protein clumps, compounding the injury. Perhaps most damaging, inflammatory microglia can convert nearby support cells called astrocytes into a toxic form that accelerates motor neuron death through excessive synaptic pruning and tissue inflammation.
The numbers from lab studies are striking. In mouse models of ALS, blocking a specific inflammatory pathway in microglia delayed disease progression by 47%. When the same pathway was artificially activated in normal, healthy microglia, motor neuron survival dropped by 50% and muscle wasting followed, even without any ALS-related genetic mutation present. This suggests that microglial inflammation alone can drive motor neuron damage.
Blood tests in ALS patients reflect this widespread immune activation. Compared to healthy people, ALS patients consistently show elevated levels of inflammatory markers including IL-6, TNF, and IL-1β. Their neutrophil-to-lymphocyte ratio is elevated, a general sign of systemic inflammation. They also show shifts in immune cell populations: increases in certain pro-inflammatory T cells and natural killer cells, with decreases in regulatory T cells that normally keep inflammation in check. Higher levels of C-reactive protein, a common marker of inflammation, correlate with faster functional decline and higher mortality risk.
A Protein That Triggers an Immune Response
A 2025 study published in Nature found something that pushes the autoimmune question further into gray territory. Researchers discovered that ALS patients mount an immune response against the C9orf72 protein, one of the most important proteins in ALS genetics. Mutations in the gene that makes this protein are the most common known genetic cause of ALS.
The study mapped the specific parts of the C9orf72 protein that the immune system recognizes and showed that a type of immune cell called CD4+ T cells drives the response. Patients carrying C9orf72 mutations had roughly six times more T cell reactivity against this protein than ALS patients with other genetic mutations. The researchers hypothesize that reduced expression of C9orf72 in certain immune cells leads to a loss of the body’s normal tolerance for the protein, essentially causing the immune system to treat it as foreign.
Interestingly, the immune response wasn’t purely harmful. Patients whose T cells released more of an anti-inflammatory signaling molecule called IL-10 in response to C9orf72 had significantly longer predicted survival times. This hints that some immune responses in ALS may be protective rather than destructive, a complexity that makes the disease harder to categorize neatly.
Antibodies in ALS Patients
Another line of evidence involves antibodies. Over 55% of ALS patients tested positive for antibodies against a protein from an ancient retrovirus embedded in human DNA, compared to about 21% of healthy people and 13% of people with MS. These retroviruses, which have been dormant in the human genome for millennia, appear to be reactivated in the brains of people with ALS. The reactivation of the viral protein seems to trigger antibody production.
Rather than being harmful, higher levels of these antibodies were associated with better survival and improved outcomes. This suggests the antibody response may represent the body’s attempt to fight the disease rather than cause it. Researchers are now exploring whether synthetic versions of these antibodies could be developed as a treatment.
How ALS Differs From Autoimmune Diseases
Several features separate ALS from diseases like MS, lupus, or rheumatoid arthritis. In autoimmune diseases, the immune attack is the primary event. Remove the immune attack (with immunosuppressive drugs, for example) and the tissue damage slows or stops. In ALS, immunosuppressive therapies have generally failed to halt progression, which supports the idea that neurodegeneration is the primary driver.
The pattern of damage is also different. In ALS, the largest nerve fibers are lost first, while in MS, the smallest fibers are preferentially affected. ALS produces characteristic TDP-43 protein clumps inside motor neurons, a signature of cell-internal dysfunction that has no equivalent in MS lesions. These features point to a disease that starts inside the neuron and spreads outward, rather than one imposed on the neuron from outside by immune cells.
The immune changes in ALS also look different from those in classic autoimmune diseases. ALS patients show decreases in the regulatory T cells that normally prevent autoimmunity, but they also show a complex mix of pro-inflammatory and anti-inflammatory responses that don’t fit the typical autoimmune pattern of a single, targeted immune attack against one tissue type.
What This Means for Treatment
The immune system’s involvement in ALS has opened real therapeutic avenues, even if ALS isn’t autoimmune in the traditional sense. Treatments that modulate inflammation rather than suppress the entire immune system are being actively tested. The discovery that blocking specific microglial inflammatory pathways can slow progression in animal models gives researchers precise targets. The finding that certain immune responses (like the antibodies against reactivated viral proteins) may be protective adds another dimension: future treatments might aim to boost helpful immune activity while dampening harmful inflammation.
For now, the most accurate way to describe ALS is as a neurodegenerative disease with a substantial neuroinflammatory component. The immune system doesn’t cause ALS in the way it causes MS or lupus, but it significantly shapes how fast the disease progresses and how much damage it does along the way.