Multiple sclerosis (MS) is a chronic autoimmune disorder affecting the central nervous system, including the brain and spinal cord. The immune system, designed to defend the body, mistakenly attacks healthy tissue, leading to an immune system that is misdirected and overly active against the self. MS itself does not inherently compromise the ability to fight external pathogens like viruses or bacteria in the traditional sense. However, the highly effective treatments used to manage MS intentionally alter or suppress parts of the immune response, which can lead to a compromised ability to fight infections.
How MS Changes Immune Function
Multiple sclerosis is characterized by an internal immune response that targets the myelin sheath, the protective fatty layer surrounding nerve fibers in the central nervous system. This process is driven by specific immune cells, known as lymphocytes, which cross the blood-brain barrier and initiate an inflammatory attack. The disease is a problem of immune overactivity directed at the self, causing inflammation and destruction of nerve insulation and subsequent neurological symptoms.
While the immune system is hyperactive in the central nervous system, the patient’s general ability to fight everyday infections may remain mostly intact during the early, untreated stages of the disease. However, individuals with untreated MS still face a higher risk of certain infections, such as urinary tract infections, often due to issues with bladder function that can accompany disease progression.
Immune Suppression Caused by MS Treatments
Managing multiple sclerosis requires the use of Disease-Modifying Therapies (DMTs) designed to reduce the frequency and severity of immune attacks on the central nervous system. These medications achieve their goal by altering, suppressing, or removing parts of the immune system, which introduces a varying degree of immune suppression. The risk of infection is directly related to the specific drug’s mechanism of action. Newer, highly effective therapies, such as those that deplete B-cells or restrict lymphocyte recirculation, tend to carry a greater risk of induced immune suppression than older injectable therapies.
Some DMTs, like anti-CD20 monoclonal antibodies (e.g., rituximab or ocrelizumab), work by eliminating B-cells, which are a type of white blood cell responsible for producing antibodies and supporting other immune functions. While this significantly reduces the autoimmune attack, the resulting depletion of B-cells can weaken the body’s ability to mount a robust antibody response against new infections or vaccines. Other oral medications, known as sphingosine-1-phosphate (S1P) receptor modulators (e.g., fingolimod), trap lymphocytes in the lymph nodes, preventing them from circulating and causing inflammation in the brain and spinal cord. This deliberate reduction in the number of circulating defense cells lowers the body’s overall surveillance capacity, increasing vulnerability to various pathogens.
The degree of immune compromise is highly variable and depends on factors like the specific agent used, the dosage, the duration of therapy, and the patient’s other health conditions. Certain treatments can lead to a sustained reduction in specific white blood cell counts, which requires regular monitoring by the care team. For instance, the use of S1P modulators is associated with an increased risk of specific infections like herpes viruses. B-cell depleters can impair the long-term immune memory needed to fend off certain illnesses.
Strategies for Minimizing Infection Risk
Individuals taking immunosuppressive DMTs must adopt strategies to manage the heightened risk of infection that comes with treatment. Strict adherence to hygiene practices is foundational, including frequent hand washing and using hand sanitizer, especially when in public or before preparing food. Behavioral modifications, such as avoiding close contact with people who are actively sick, limit exposure to common respiratory and gastrointestinal illnesses.
Vaccination is a particularly important strategy, but it requires careful planning with the MS care team to optimize effectiveness and safety. Inactivated vaccines, which contain non-living virus or bacteria components, are generally safe to receive while on most DMTs, and annual influenza and COVID-19 vaccines are highly recommended. Live-attenuated vaccines, which contain a weakened form of the living pathogen, are usually contraindicated for individuals on immunosuppressive therapies due to the risk of the vaccine causing a full-blown infection.
The timing of non-live vaccinations is important, especially with therapies that deplete or sequester immune cells. For maximum immune response, it is often recommended to administer a vaccine weeks before starting a new DMT or during a specific window between doses of a pulsed therapy, such as anti-CD20 infusions. Patients must maintain open communication with their neurologist and primary care provider to ensure their vaccination schedule aligns with their treatment cycle.
Monitoring for early signs of infection, such as fever, persistent cough, or unusual pain, allows for prompt medical intervention. This is essential when the immune system’s response is deliberately slowed.