Immunogenicity in Biosimilars: Why Immune Responses May Differ From Reference Biologics

When a patient switches from a brand-name biologic to a biosimilar, they might wonder: Will my body react differently? It’s a valid concern. Unlike generic pills, which are chemically identical to their originals, biosimilars are made from living cells - and even tiny differences in how those cells produce the protein can change how your immune system sees it. That’s where immunogenicity comes in: the chance that your body will recognize the drug as foreign and build antibodies against it.

What Exactly Is Immunogenicity?

Immunogenicity means your immune system detects a drug as something foreign and mounts a response. This usually shows up as anti-drug antibodies (ADAs). These aren’t always bad - many people develop low-level ADAs without any symptoms. But in some cases, those antibodies can block the drug from working, or even cause serious side effects like allergic reactions or infusion reactions.

For example, the drug cetuximab - used for colorectal cancer - once caused life-threatening anaphylaxis in some patients because of a sugar molecule (galactose-α-1,3-galactose) attached to it. That sugar isn’t found in humans, so the immune system treated it like an invader. Even fully human proteins can trigger this. Why? Because the way they’re folded or modified during manufacturing creates new surfaces your immune system hasn’t seen before.

Why Biosimilars Aren’t Identical to the Original

Think of biologics like a complex origami crane. The instructions are the same - but if you fold it in a slightly different environment, with slightly different tools, the final shape can vary. Biosimilars are made in different cell lines (often Chinese hamster ovary cells vs. human cell lines), using different purification methods, and under different conditions. These differences don’t change the overall structure, but they can alter tiny details:

• Glycosylation: The sugar chains attached to the protein. Even a 5% change in sialic acid or galactose content can affect how long the drug lasts in your body - and whether immune cells notice it.
• Protein aggregates: If even 5% of the drug molecules clump together, the risk of immune response jumps 3.2 times. These clumps look like pathogens to your immune system.
• Host cell proteins: Leftover proteins from the manufacturing cells. If levels go above 100 parts per million, ADA rates spike by 87%.

These aren’t errors - they’re unavoidable byproducts of using living systems to make drugs. But they matter. A 2020 study found that differences in glycosylation patterns between reference adalimumab and its biosimilar Amgevita correlated with higher ADA rates (23.4% vs. 18.7%).

How Your Body Influences the Response

It’s not just the drug. Your biology plays a huge role. Someone with rheumatoid arthritis has a 2.3 times higher risk of developing ADAs than a healthy person. Why? Their immune system is already turned up - like a car alarm set too sensitive.

Other factors:

• Genetics: People with the HLA-DRB1*04:01 gene variant are nearly five times more likely to develop ADAs to certain biologics.
• Other meds: Taking methotrexate with a TNF inhibitor cuts ADA risk by 65%. It’s like putting a mute button on your immune system.
• Route of delivery: Injecting under the skin (subcutaneous) carries 30-50% higher immunogenicity risk than IV infusion. More exposure to immune cells near the skin.
• Dosing frequency: Weekly or monthly shots break immune tolerance faster than continuous IV infusions.

So even if two biosimilars are identical on paper, one person might react and another won’t - because their immune system is wired differently.

How Do We Measure This?

Testing for ADAs isn’t straightforward. Labs use different tools - some are more sensitive than others. One method, electrochemiluminescence (ECL), can detect ADAs in 13.1% of patients, while older tests catch only 5%. That’s why comparing studies is tricky.

Regulators demand head-to-head testing. The EMA says biosimilar trials must use the exact same assay to measure ADAs as the reference product. Otherwise, you can’t tell if a difference is real - or just a faulty test.

The FDA requires a tiered approach:

1. Screening: Does the patient have any antibodies at all?
2. Confirmation: Are those antibodies specific to the drug - or just background noise?
3. Characterization: Are they neutralizing? Do they block the drug’s action?

Neutralizing antibodies are the real concern. They don’t just bind to the drug - they stop it from working. That’s why cell-based assays are preferred: they show whether the drug still functions in a live system, even if binding looks normal.

Real-World Evidence: What Do We Actually See?

Here’s the twist: most real-world data shows little to no difference.

The NOR-SWITCH trial followed 481 patients who switched from infliximab to its biosimilar. ADA rates rose slightly - 8.5% to 11.2% - but no one lost clinical response. Another study of 1,247 rheumatoid arthritis patients found identical ADA rates: 12.3% on the original, 11.8% on the biosimilar.

But there are exceptions. The Danish Biologics Registry found a statistically higher ADA rate (23.4%) with Amgevita compared to Humira (18.7%). And on patient forums, stories vary wildly. One Reddit user reported severe injection site reactions after switching to a biosimilar etanercept. Another said they switched five times between brands and biosimilars - and felt zero difference.

A 2022 survey of 347 rheumatologists showed 68% think immunogenicity fears are overblown. But 22% say they’ve seen real clinical differences - like loss of response or new rashes.

What About Formulation Differences?

Even small changes in the liquid around the drug matter. Rituxan (originator) uses polysorbate 20 as a stabilizer. Rixathon (biosimilar) uses polysorbate 80. Sounds minor - but these surfactants can affect protein stability. One can promote aggregation. The other might not. And aggregated protein = higher ADA risk.

This is why regulators now require detailed formulation comparisons. You can’t just match the protein - you have to match the whole package.

Where Are We Headed?

The future lies in precision. Scientists are now using mass spectrometry to map every sugar and amino acid on a biologic molecule - down to 99.5% accuracy. By 2027, this could mean biosimilars are virtually indistinguishable at the molecular level.

Some labs are combining proteomics (protein structure), glycomics (sugar patterns), and immunomics (how immune cells respond) into one risk score. Projects like those at UCSF are already testing this in trials (NCT04875682, NCT05123456).

For now, the message is clear: most biosimilars are safe and effective. But immunogenicity isn’t a myth. It’s a real, measurable, and sometimes clinically relevant risk - especially for patients with certain genes, diseases, or dosing schedules. The goal isn’t to avoid biosimilars. It’s to understand them deeply - and match the right one to the right patient.