The science behind TYSABRI

Your immune system and relapsing MS

B cells and T cells are critical in helping your body defend itself against threats like viruses and bacteria.

In relapsing MS, some of these cells may cross into your central nervous system (CNS) and go “rogue” by mistakenly attacking your own nerves. This can lead to damage to your CNS and cause symptoms of relapsing MS.

TYSABRI and relapsing MS

We don’t fully understand how TYSABRI works, but it’s thought to play a role in preventing rogue immune cells from entering your CNS and mistakenly attacking your nerve cells.

Keeping B and T cells in your bloodstream and blocking them from entering your CNS, TYSABRI may help reduce damage to your nerve cells.

TYSABRI does not destroy your B cells and T cells.

Take a look at the video below to find out more about how TYSABRI is thought to work!

View transcript

Narrator: Relapsing multiple sclerosis is a progressive disease with no cure, which means that it can get worse over time. This is one reason why continuous treatment is so important.

While no one knows how TYSABRI works exactly, it is believed to help prevent harmful immune cells from crossing into the central nervous system, or CNS, in the first place.  Reducing the number of harmful immune cells in the brain may reduce damage and inflammation.

If you are interested in learning more about relapsing MS or TYSABRI, please talk to your healthcare provider.

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Talking to your healthcare team

What is TYSABRI® (natalizumab)?

TYSABRI® (natalizumab) is a prescription medicine used to treat relapsing forms of multiple sclerosis (MS), to include clinically isolated syndrome, relapsing-remitting disease, and active secondary progressive disease. TYSABRI increases the risk of progressive multifocal leukoencephalopathy (PML). When starting and continuing treatment with TYSABRI, it is important to discuss with your doctor whether the expected benefit of TYSABRI is enough to outweigh this risk. 

It is not known if TYSABRI is safe and effective in children under 18 years of age.

IMPORTANT SAFETY INFORMATION

What is the most important information I should know about TYSABRI?

TYSABRI increases your risk of getting a rare brain infection—called progressive multifocal leukoencephalopathy (PML)—that usually leads to death or severe disability.

  • There is no known treatment, prevention, or cure for PML
  • You should not take certain medicines that weaken your immune system at the same time you are taking TYSABRI. Even if you use TYSABRI alone to treat your MS, you can still get PML
  • Your risk of getting PML is higher if you:
    • have been infected by the John Cunningham Virus (JCV). JCV is a common virus that can cause PML in people who have weakened immune systems, such as people taking TYSABRI. Before or while you receive TYSABRI, your doctor may do a blood test to check if you have been infected by JCV
    • have received TYSABRI for a long time, especially for longer than 2 years
    • have received certain medicines that can weaken your immune system before you start receiving TYSABRI
  • Your risk of getting PML is greatest if you have all 3 risk factors listed above. There may be other risk factors that have not yet been identified
  • Patients who are anti-JCV antibody negative are still at risk for the development of PML due to the potential for a new JCV infection or a false negative test result. Therefore, patients with a negative test result should be retested periodically
  • While you receive TYSABRI, and for 6 months after you stop receiving TYSABRI, it is important that you call your doctor right away if you have any new or worsening medical problems (such as problems with your thinking, eyesight, balance, or strength; weakness on 1 side of your body; and using your arms and legs) that have lasted several days. Tell all of your doctors that you are getting treatment with TYSABRI
  • Because of your risk of getting PML while you receive TYSABRI, TYSABRI is available only through a restricted distribution program called the TOUCH® Prescribing Program

Who should not receive TYSABRI?

Do not receive TYSABRI if you have PML or are allergic to natalizumab or any of the ingredients in TYSABRI.

What should I tell my doctor before receiving each dose of TYSABRI?

Before receiving TYSABRI, it is important to tell your doctor:

  • If you have a medical condition that can weaken your immune system, such as HIV infection or AIDS, leukemia or lymphoma, organ transplant, or others, or if you have any new or worsening medical problems that have lasted several days
  • If you are pregnant or plan to become pregnant. TYSABRI may cause low platelets (thrombocytopenia), and low red blood cells (anemia) in your newborn baby if you take TYSABRI while you are pregnant. It is not known if TYSABRI can cause birth defects
  • If you are breastfeeding or plan to breastfeed. TYSABRI can pass into your breast milk. It is not known if TYSABRI that passes into your breast milk can harm your baby. Talk to your doctor about the best way to feed your baby while you receive TYSABRI
  • About all of the medicines and supplements you take, especially medicines that can weaken your immune system. If you are not sure, ask your doctor

What are the possible side effects of TYSABRI?

TYSABRI can cause serious side effects. If you have any of the symptoms listed below, call your doctor right away:

  • Herpes infections. Increased risk of infection of the brain or the covering of your brain and spinal cord (encephalitis or meningitis) caused by herpes viruses that may lead to death. Symptoms include sudden fever, severe headache, or confusion. Infection of the eye caused by herpes viruses leading to blindness in some patients has occurred. Call your doctor if you have changes in vision, redness, or eye pain
  • Liver damage. Symptoms include yellowing of the skin and eyes (jaundice), unusual darkening of the urine, nausea, feeling tired or weak, or vomiting
  • Allergic reactions (e.g., hives, itching, trouble breathing, chest pain, dizziness, wheezing, chills, rash, nausea, flushing of skin, low blood pressure), including serious allergic reactions (e.g., anaphylaxis). Serious allergic reactions usually happen within 2 hours of the start of the infusion, but they can happen any time after receiving TYSABRI
  • Weakened immune system. TYSABRI may increase your risk of getting an unusual or serious infection
  • Low platelet counts. TYSABRI may cause the number of platelets in your blood to be reduced. Symptoms include easy bruising, small scattered spots on your skin that are red, pink or purple, heavier menstrual periods than are normal, bleeding from your gums or nose that is new or takes longer than usual to stop, or bleeding from a cut that is hard to stop

The most common side effects of TYSABRI are:

  • Headache, feeling tired, urinary tract infection, joint pain, lung infection, depression, pain in your arms or legs, diarrhea, vaginitis, rash, nose and throat infections, nausea, stomach area pain. If you experience any side effect that bothers you or does not go away, tell your doctor

These are not all of the possible side effects of TYSABRI. For more information, ask your doctor. To report side effects to FDA, please call 1-800-FDA-1088.

Please see full Prescribing Information, including Boxed Warning and Medication Guide.

This information does not take the place of talking with your doctor about your medical condition or your treatment.

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About TYSABRI
Why TYSABRI? Safety considerations Long-term study Taking TYSABRI How TYSABRI works
Starting TYSABRI
Talking to your doctor The TOUCH Program Find an infusion site Infusion reminders
What is relapsing MS?
Learn about RMS
Cost Info & Support
Biogen Support Services Support Coordinators Financial & insurance assistance FAQs about TYSABRI Care partner resources

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©2025 Biogen. All rights reserved.
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