ALS is not classified as an autoimmune disease. It is a neurodegenerative disease, meaning it is driven by the progressive death of motor neurons rather than by the immune system attacking healthy tissue. However, the relationship between ALS and the immune system is far more complex than that simple classification suggests, and growing evidence points to immune dysfunction as a significant factor in how quickly the disease progresses.
Why ALS Is Not Autoimmune
In a true autoimmune disease like multiple sclerosis, the immune system mistakenly identifies healthy cells as threats and directly attacks them. The damage is primarily caused by immune activity, and suppressing the immune system can slow or stop the disease. ALS does not work this way. The core problem in ALS is the degeneration of upper and lower motor neurons, the nerve cells that control voluntary movement. There are no clear genetic immune markers (called human lymphocyte antigen associations) linking ALS to autoimmune patterns, and standard immune-suppressing treatments have not been effective against it.
When doctors diagnose ALS, they use criteria that focus on progressive motor dysfunction and evidence of both upper and lower motor neuron damage. Immune-related blood tests are not part of the positive diagnosis. Instead, they are used to rule out conditions that can mimic ALS, such as multifocal motor neuropathy, myasthenia gravis, and inclusion body myositis, all of which involve specific antibodies that can be detected through testing.
The Immune System’s Role in ALS Progression
Even though ALS is not autoimmune, the immune system is deeply involved in the disease. As motor neurons degenerate, the brain and spinal cord’s resident immune cells (called microglia) become activated, along with supporting cells called astrocytes. This activation triggers widespread inflammation in the nervous system. The open question, and it is one researchers are actively debating, is whether this immune response merely reacts to neuronal damage or actively drives the disease forward.
Recent evidence increasingly supports the latter. A study tracking immune changes in ALS patients found that immune dysregulation occurs before clinical decline, not just after it, suggesting the immune system is a causal factor in disease progression rather than a bystander. Multiple types of immune cells, including neutrophils, natural killer cells, monocytes, and T cells, have been found accumulating in the central nervous system of people with ALS.
What makes this especially nuanced is that not all immune activity in ALS is harmful. Research in mouse models of ALS revealed something unexpected: T cells that infiltrate the spinal cord during disease progression actually appear to be protective. When researchers bred ALS mice to lack T cells, the disease progressed significantly faster, particularly in the later stages. The T cells were promoting a protective response in microglia by releasing an anti-inflammatory signal, and without that signal, motor neuron loss accelerated. Astrocytes, by contrast, appeared to be more consistently toxic. Fluid from ALS-affected astrocytes was directly harmful to motor neurons in laboratory conditions, while fluid from activated microglia showed minimal effect.
Inflammation Markers Predict Disease Speed
One of the most practical connections between immune function and ALS is that simple blood-based inflammation markers can help predict how quickly the disease will progress. Researchers have found that the ratio of neutrophils to lymphocytes in a patient’s blood correlates strongly with progression rate. Patients with higher ratios of these inflammatory cells were more than four times as likely to be fast progressors (losing more than one functional point per month) compared to those with lower ratios. These same markers independently predicted survival time even after accounting for age, sex, symptom onset location, body weight, and medication use.
This means that while ALS is not caused by immune dysfunction in the way autoimmune diseases are, the state of a patient’s immune system meaningfully shapes their prognosis.
The C9orf72 Gene and Immune Dysfunction
The strongest genetic link between ALS and the immune system involves mutations in the C9orf72 gene, which is the most common genetic mutation found in ALS patients. Researchers at Harvard discovered that this gene does not only affect neurons. Mice lacking a functional copy of C9orf72 developed abnormally large spleens, livers, and lymph nodes, all signs of immune system overactivation. These mice became sick and died, and the research team noted that the gene’s effect on cells involved in autoimmunity was “unexpected.”
When these same mice received bone marrow transplants from healthy donors, replacing their dysfunctional immune cells, they lived an average of 43 days longer, weighed more, and maintained healthier blood counts. This finding suggests that in at least some forms of ALS, immune dysfunction driven by the same genetic mutation that affects neurons may contribute independently to disease severity.
Complement System Activity in ALS
Another piece of the puzzle involves the complement system, a part of the innate immune response that helps clear damaged cells. In ALS patients, complement activity is elevated both in blood plasma and in spinal fluid compared to healthy individuals and even compared to people with other neurological diseases. Plasma from ALS patients is cytotoxic: when mixed with healthy red blood cells or nerve tissue in laboratory settings, it causes damage. The degree of this toxic activity correlates with complement levels.
Some researchers have proposed that ALS may partially or fully involve complement activation, and that the disease may begin at the connections between nerves and muscles (neuromuscular junctions) rather than in the brain and spinal cord. This would reframe ALS as more of a peripheral immune-driven process than a purely central nervous system disease, though this remains a hypothesis under investigation.
Immune-Targeting Treatments Under Study
Because of the growing evidence that immune dysregulation worsens ALS, several clinical trials are now testing therapies that modulate the immune system rather than suppress it outright. One active trial at Johns Hopkins is evaluating a combination therapy designed to boost a specific type of protective immune cell (regulatory T cells) while simultaneously dampening the inflammatory response from activated immune cells. This dual approach reflects the nuanced understanding that some immune activity in ALS is harmful while other immune activity is protective, and that blunt immunosuppression is unlikely to help.
This marks a shift from earlier decades when researchers tested broad immune-suppressing drugs for ALS and saw no benefit. The newer strategies are built on the recognition that the immune system in ALS is dysregulated, not simply overactive, and that restoring balance may slow progression even if it cannot address the underlying cause of motor neuron death.