Fasenra (benralizumab) is not a traditional immunosuppressant. It does not broadly suppress your immune system the way corticosteroids or drugs like azathioprine do. Instead, it targets and eliminates one specific type of white blood cell, eosinophils, while leaving the rest of your immune defenses largely intact. That distinction matters because it changes the risk profile significantly compared to the medications many severe asthma patients are trying to reduce or replace.
How Fasenra Works
Fasenra is a monoclonal antibody, a lab-made protein designed to lock onto a very specific target. In this case, it binds to the receptor for interleukin-5 (IL-5) on the surface of eosinophils. IL-5 is the signal that tells your body to produce and activate eosinophils, so by blocking that receptor, Fasenra essentially flags those cells for destruction.
What makes Fasenra different from other asthma biologics is how it destroys eosinophils. Rather than just blocking the signal that keeps eosinophils alive, it recruits natural killer (NK) cells, a type of immune cell your body already uses to eliminate threats. Fasenra physically brings NK cells and eosinophils into close contact, and the NK cells then release proteins that kill the eosinophils directly. This process, called antibody-dependent cell-mediated cytotoxicity, results in near-complete depletion of eosinophils from both the blood and tissues. It’s a targeted elimination, not a system-wide shutdown.
Why It’s Not a Broad Immunosuppressant
Traditional immunosuppressants like systemic corticosteroids (prednisone, for example) dampen the activity of many different immune cells at once. They reduce inflammation effectively, but they also impair your body’s ability to fight bacteria, viruses, and fungi. That’s why long-term steroid use comes with well-known risks: frequent infections, bone loss, weight gain, diabetes, and more. Other immunosuppressants used in severe inflammatory conditions, such as cyclophosphamide and azathioprine, carry similar broad toxicity.
Fasenra works on a much narrower slice of the immune system. It does not suppress T cells, B cells, or neutrophils, the immune cells responsible for fighting most infections. It activates certain parts of the immune system (specifically NK cells) while depleting only eosinophils. That’s why the medical classification is better described as “immunomodulator” or “targeted biologic” rather than immunosuppressant.
That said, eosinophils aren’t useless bystanders. They play roles in host defense against certain parasites, contribute to tissue repair, and have some involvement in immune regulation. Eliminating them entirely isn’t without biological consequences, even if those consequences are far milder than broad immune suppression.
Effects on Other Immune Cells
Research has found that Fasenra does produce some changes beyond eosinophil depletion, though the effects are subtle and may actually be beneficial. Over 24 months of treatment, patients showed a reduction in a type of T cell associated with inflammation (effector memory T cells) and an increase in regulatory T cells, which help keep immune responses in check and prevent overreaction. This rebalancing effect is considered a positive shift in patients with severe eosinophilic asthma, where the immune system is already in a state of chronic overactivation.
These changes don’t amount to immune suppression in any clinically meaningful sense. Your body retains its ability to mount responses against bacteria, viruses, and other common pathogens.
Infection Risk in Practice
Clinical trials have not shown a significant increase in serious infections with Fasenra compared to placebo. The rates of common respiratory infections like colds and sinus infections are similar between treated patients and those receiving a dummy injection. This stands in sharp contrast to systemic corticosteroids, where infection risk climbs steadily with dose and duration.
There is one important exception: parasitic infections. Eosinophils are one of the body’s primary weapons against helminths (parasitic worms). Because Fasenra depletes eosinophils so thoroughly, the FDA labeling requires that any existing parasitic infection be treated and resolved before starting the medication. If you develop a parasitic infection while on Fasenra and it doesn’t respond to treatment, the drug should be stopped until the infection clears. For most people living in areas where parasitic infections are uncommon, this is a minimal practical concern, but it’s worth knowing if you travel to regions where these infections are more common.
How This Compares to Steroids It Replaces
One of Fasenra’s main benefits for people with severe eosinophilic asthma is its potential to reduce or eliminate the need for daily oral corticosteroids. Many patients on Fasenra are able to taper their steroid doses significantly. From an immune standpoint, this is often a net gain: you’re trading a drug that suppresses your entire immune system for one that targets a single cell type. The overall burden on your immune defenses goes down, not up.
This is a key point that gets lost in the “is it an immunosuppressant?” question. If Fasenra allows you to stop taking prednisone every day, your immune function will likely improve over time as the broad suppressive effects of steroids wear off, even though Fasenra itself is eliminating your eosinophils. The tradeoff favors the targeted approach for most patients.
What This Means for You
If you’re considering Fasenra or already taking it, you don’t need to take the same precautions you would with a true immunosuppressant. You’re not at elevated risk for opportunistic infections the way someone on high-dose steroids or chemotherapy would be. You don’t need to avoid crowds or take special measures during cold and flu season beyond what’s normally recommended.
You should, however, be aware that your body will have essentially zero circulating eosinophils while on the medication. For the vast majority of everyday immune challenges, this doesn’t matter. But if you’re planning travel to tropical regions where parasitic infections are a risk, or if you develop unexplained symptoms that could suggest a parasitic infection, bring up your Fasenra use with your doctor so it can be factored into your care.