Ocrevus (ocrelizumab) works by selectively destroying a specific type of immune cell called B cells, which play a central role in the nerve damage caused by multiple sclerosis. It is the first therapy approved for both relapsing and primary progressive forms of MS, and it’s given as an intravenous infusion every six months.
How Ocrevus Targets B Cells
Ocrevus is a monoclonal antibody, meaning it’s a lab-engineered protein designed to lock onto one specific target. In this case, that target is CD20, a protein found on the surface of B cells. B cells are a type of white blood cell that, in MS, mistakenly attack the protective coating (myelin) around nerve fibers in the brain and spinal cord. When Ocrevus binds to CD20, it flags those B cells for destruction by the rest of the immune system.
The drug depletes nearly all circulating B cells in the blood, along with a subset of T cells that also carry the CD20 marker. However, the depletion isn’t uniform throughout the body. B cells in lymph nodes and in inflamed areas of the central nervous system are not eliminated to the same extent. Importantly, plasma cells, which are mature immune cells responsible for producing long-term antibodies against infections you’ve already fought off, don’t carry CD20 and are largely spared. This is why Ocrevus can suppress the harmful immune activity in MS while still preserving some of your existing immune protection.
What Happens During Relapsing MS
The strongest evidence for Ocrevus in relapsing MS comes from two large clinical trials called OPERA I and OPERA II. In both studies, patients receiving Ocrevus had roughly a 46 to 47% lower relapse rate compared to those on an older injectable MS therapy. Ocrevus also significantly reduced the number of new brain lesions visible on MRI, including both active inflammatory lesions and the “black holes” that represent more permanent tissue damage.
For people with relapsing MS, fewer relapses and fewer new lesions generally translate to slower accumulation of disability over time. This is the core goal of any disease-modifying MS therapy: keep the immune system from launching new attacks on the brain and spinal cord.
How It Works in Primary Progressive MS
Primary progressive MS (PPMS) is a form of the disease where disability gradually worsens from the start, without the clear relapses and remissions that characterize the relapsing form. Before Ocrevus, no therapy had been approved specifically for PPMS.
In the ORATORIO trial, Ocrevus reduced the risk of confirmed disability progression by about 24 to 25% compared to placebo. That was measured at both 12-week and 24-week confirmation points, meaning the benefit held up over time rather than being a temporary fluctuation. While a 24% reduction is more modest than what’s seen in relapsing MS, it represented a meaningful shift for a population that previously had no approved treatment options.
The Infusion Schedule
Your first course of Ocrevus is split into two smaller infusions of 300 mg each, given two weeks apart. After that, you receive a single 600 mg infusion every six months. Each infusion is given at a clinic or infusion center, and you’ll typically stay for an observation period afterward.
Before each infusion, you’ll receive pre-medications, usually a corticosteroid and an antihistamine, to reduce the chance of an infusion reaction. Your care team will also check for any active infections before proceeding, since the drug suppresses part of your immune system.
Infusion Reactions and Side Effects
Infusion reactions are the most common side effect, occurring in 34 to 40% of patients in clinical trials even with pre-medication. They’re most likely during the first infusion and tend to become less frequent with subsequent doses. Symptoms can include itching, flushing, headache, throat irritation, dizziness, nausea, and a drop in blood pressure. Most reactions are mild to moderate and can be managed by slowing or pausing the infusion.
Because Ocrevus depletes B cells, it does lower your immune defenses to some degree. Your doctor will check your immunoglobulin levels (the antibodies your body uses to fight infections) before starting treatment. If those levels are already low, you may need further evaluation before beginning the drug.
Screenings Before Starting Treatment
Before your first infusion, several screening steps are required. You’ll be tested for hepatitis B, because Ocrevus can reactivate the virus in people who carry it. Active hepatitis B infection is a firm contraindication, meaning you cannot receive the drug. If you have evidence of past exposure, your care team will involve a liver specialist to weigh the risks.
Vaccinations also need to be addressed ahead of time. Live vaccines should be given at least four weeks before starting Ocrevus, and non-live vaccines at least two weeks before. Once you’re on the drug, live vaccines are not recommended because your depleted B cells may not be able to mount a safe response. This applies for the entire duration of treatment and for some time after stopping.
What Happens When You Stop Ocrevus
B cells don’t bounce back immediately after your last infusion. At six months after the final dose, only about 3 to 5% of patients show early signs of B-cell recovery in their blood. By nine months, roughly half of patients have detectable B-cell repopulation. By 12 months, 85 to 90% show recovery. Full return to normal B-cell levels takes a median of 60 to 72 weeks.
This slow recovery timeline matters for practical decisions like vaccination planning and pregnancy planning. If you need to receive a live vaccine or are considering stopping treatment for any reason, your care team will factor in this recovery window to determine the safest timing.