What Is Biotherapy and How Does It Work?

Biotherapy is the use of living organisms, substances derived from living organisms, or laboratory-made versions of those substances to treat disease. The term covers an enormous range of treatments, from antibodies grown in cell culture to genetically engineered immune cells to viruses designed to destroy tumors. What ties them together is their biological origin and their reliance on the body’s own systems, especially the immune system, to produce a therapeutic effect. The field has grown from a single daring experiment in the 1890s into one of the fastest-moving areas of modern medicine.

Where the Idea Began

The concept of using biology against disease is older than most people realize. In 1891, a New York surgeon named William B. Coley injected streptococcal bacteria directly into a patient with inoperable cancer, reasoning that the resulting infection might also shrink the tumor. It worked. Over the next four decades, Coley treated more than a thousand cancer patients with bacteria or bacterial products, which became known as Coley’s Toxins.¹ The approach was crude and unpredictable, and it fell out of favor as surgery and radiation improved, but the underlying insight was sound: the immune system can be provoked into attacking cancer. That principle now drives a multi-billion-dollar class of drugs.

How Biologics Differ from Conventional Drugs

Most traditional medications are small molecules, compact chemical compounds synthesized in a lab. Aspirin, metformin, and statins all fall into this category. Biologics are fundamentally different. They are derived from living cells or biological processes and tend to be large, complex molecules made of proteins, carbohydrates, nucleic acids, or combinations of these. In terms of sheer size, a typical small-molecule drug weighs less than 1 kilodalton, while biologics are usually larger, sometimes much larger.²

That size and complexity matter for several practical reasons. Small molecules are relatively simple structures that can be copied precisely by different manufacturers, and they rarely provoke an immune reaction on their own. Biologics are sensitive to their manufacturing process and starting materials. Even slight changes in how a biologic is produced can alter its structure and, potentially, its behavior in the body. Their structural complexity also means they can sometimes trigger immune responses, a consideration that does not typically arise with a conventional pill.

Monoclonal Antibodies

Monoclonal antibodies are among the most widely used biotherapies. They are lab-made proteins designed to bind to a specific target on a cell surface, the way a key fits one lock. Once an antibody latches onto its target, it can work through several mechanisms. It may recruit immune cells such as natural killer cells, T cells, and macrophages to destroy the tagged cell, a process driven by antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity.³ In other words, the antibody flags a dangerous cell and the immune system does the killing.

Natural killer cells and macrophages play especially important roles here. Natural killer cells can directly destroy antibody-coated cells, while macrophages engulf and digest them. The complement system, a cascade of proteins in the blood, can also punch holes in the membrane of a tagged cell. How effectively each of these pathways contributes depends on the specific antibody, the target it recognizes, and the type of tumor or disease involved.⁴

A more recent twist on the monoclonal antibody concept is the antibody-drug conjugate. These are antibodies chemically linked to a potent toxic payload via a molecular tether called a linker. The antibody delivers its cargo specifically to cells expressing a particular surface marker, sparing healthy tissue from the worst effects of the toxin. This represents a meaningful step beyond traditional chemotherapy, which tends to affect dividing cells indiscriminately.⁵

Checkpoint Inhibitors

Your immune system has built-in brakes that prevent T cells from attacking healthy tissue. Cancer cells exploit these brakes. They display surface proteins that press the “off switch” on T cells patrolling nearby, essentially hiding in plain sight. Checkpoint inhibitors are biotherapies that block those off switches, freeing the immune system to recognize and attack the tumor.

Two of the most studied checkpoints are CTLA-4 and PD-1. CTLA-4 normally dials down T cell activity early in an immune response, mostly in lymph nodes, while PD-1 suppresses T cells later, in the tissues where the actual fight is happening. Drugs that block either pathway have produced remarkable results in melanoma, lung cancer, and a growing list of other cancers.⁶ These therapies do not work for everyone, but when they do work, the responses can be durable in ways that were almost unheard of with older chemotherapy regimens.

CAR-T Cell Therapy

If checkpoint inhibitors take the brakes off your existing immune cells, CAR-T therapy goes further: it reengineers your own T cells into targeted cancer killers. The process starts by drawing blood from the patient and isolating T cells. Those cells are then genetically modified in the lab to carry a chimeric antigen receptor, a synthetic molecule on the cell surface that lets the T cell recognize and lock onto a specific protein found on cancer cells. The modified cells are multiplied into the hundreds of millions and then infused back into the patient.⁷

The results in blood cancers have been striking, with patients who had exhausted all other options sometimes achieving complete remissions. Researchers continue to refine the approach, modifying the receptor design and combining it with other agents to boost effectiveness.⁸ A persistent challenge is that CAR-T therapy has been far less effective against solid tumors than against blood cancers. The physical barriers and immunosuppressive environment inside a solid tumor make it difficult for the engineered cells to infiltrate and survive.

Cytokines and Cancer Vaccines

Not every biotherapy involves engineering cells or designing antibodies. Cytokines are naturally occurring signaling molecules that immune cells use to communicate. Giving patients synthetic versions of certain cytokines can amplify the immune response against cancer. Interferon alpha, one of the first cytokines produced through recombinant DNA technology, has been used for decades and regulates both cell growth and immune function.⁹ Cytokine therapy remains an active area of research, contributing to current clinical trials even as newer approaches steal the spotlight.¹⁰

Cancer vaccines represent another branch of biotherapy. Unlike preventive vaccines for infections, therapeutic cancer vaccines are designed to train the immune system to recognize and attack a tumor that already exists. One promising avenue uses mRNA technology. By introducing mRNA encoding tumor-specific antigens into the body, these vaccines prompt cells to produce the antigen, which in turn triggers an immune response targeting cells that carry the same marker.¹¹ The approach gained massive public visibility during the COVID-19 pandemic, but the underlying concept of using mRNA to direct the body’s own protein-making machinery was already being explored in oncology well before that.

Oncolytic Viruses

If the idea of injecting bacteria into tumors sounded bold in 1891, modern researchers have raised the stakes by using viruses. Oncolytic viruses are engineered or naturally selected to infect and replicate specifically within tumor cells, bursting them open while leaving healthy tissue alone. When the infected cancer cells die, they release signals that draw the immune system’s attention to the tumor site, effectively turning each dying cell into a beacon.¹² The approach combines direct tumor destruction with immune activation, and it is being tested both alone and in combination with checkpoint inhibitors.

Biotherapy Beyond Cancer

Although cancer dominates the conversation, biotherapy extends well beyond oncology. Autoimmune diseases like rheumatoid arthritis were among the first conditions to benefit from biologic agents. In rheumatoid arthritis, overactive T cells drive the production of inflammatory molecules, especially tumor necrosis factor, which fuels joint inflammation and destruction.¹³ Drugs that block TNF, such as infliximab, etanercept, and adalimumab, were the first biologic therapies licensed for the disease and remain widely used. Other biologic targets, including interleukin-6 and interleukin-1, have since been validated, giving clinicians multiple options when one agent does not work well enough.¹⁴

Gene therapy is another form of biotherapy making real headway. Using viral vectors, especially adeno-associated virus, researchers can deliver corrective DNA sequences directly into a patient’s cells. These engineered viral shells carry no viral genes, only the therapeutic DNA, making them one of the safest gene-delivery tools developed so far.¹⁵ Gene therapies are now approved for a handful of inherited conditions, with many more in clinical trials.

Stem cell therapy for neurodegenerative diseases is still largely experimental, but it represents one of the most hopeful applications. The idea is to replace damaged neurons with new cells derived from stem cells, or at least to create a local environment that protects surviving neurons from further harm.¹⁶ The science here is less mature than in oncology, but two decades of research have demonstrated that the concept holds real potential.

How Biologics Are Manufactured

Making a biologic is nothing like making a pill. Most therapeutic proteins, monoclonal antibodies in particular, are produced inside living cells grown in large bioreactors. The workhorse of the industry is the Chinese hamster ovary cell line, which has been the dominant production platform for decades.¹⁷ These cells are genetically modified to produce the desired protein, then cultured under carefully controlled conditions. Advances in cell-line engineering and media optimization have pushed yields past 10 grams per liter of culture, but the process remains technically demanding.¹⁸

The sensitivity of biologics to their manufacturing process creates a challenge that small-molecule drugs never face. When a conventional drug goes off patent, generic manufacturers can reproduce the exact chemical compound. With biologics, no copy is truly identical to the original. These near-copies, called biosimilars, must undergo rigorous comparison testing to demonstrate that they are analytically, pharmacologically, and clinically similar to the reference product.¹⁹ The approval pathway is more complex and expensive than for generic pills, which is one reason biologic drugs remain costly even after their original patents expire.

Safety Concerns

Because biotherapies work by manipulating the immune system, they come with a distinctive set of risks. One of the most serious is cytokine release syndrome, an exaggerated inflammatory response that can occur when immune cells are suddenly activated on a massive scale. Symptoms range from fever and low blood pressure in mild cases to organ failure in severe ones. In a Swedish observational study of over 2,600 patients treated with checkpoint inhibitors, the risk of cytokine release syndrome was roughly one percent, higher than earlier estimates had suggested. Most episodes were mild, and retreatment after a mild episode was generally safe, though fatal cases did occur.²⁰

Identifying which patients are at risk of severe reactions is an active area of research. Some clinical markers at the time a patient first develops fever, including lower platelet counts and higher urea levels, have been associated with more dangerous episodes.²¹ Standard management includes corticosteroids and, in more severe cases, tocilizumab, though outcomes remain poor once the syndrome escalates to the highest grades.

Predicting Who Will Respond

One frustration with biotherapy, particularly in cancer, is that it works spectacularly for some patients and not at all for others. A tumor’s expression of PD-L1, the protein that checkpoint inhibitors are designed to counteract, is one of the most commonly used biomarkers for predicting response. Regulatory agencies have approved companion diagnostic tests that measure PD-L1 levels on tumors to help guide treatment decisions.²² But PD-L1 alone is an imperfect predictor. Some patients with low PD-L1 expression still respond, and some with high expression do not. Combining PD-L1 measurement with other markers and advanced genomic techniques is expected to sharpen patient selection over time, but for now, predicting response remains more art than science.

Cost and Access

Biotherapies are expensive, and the reasons go beyond corporate pricing strategies. Manufacturing cell and gene therapies requires significant upfront investment in specialized facilities and quality-control systems, and the production process itself is costly and technically challenging.²³ For patient-specific treatments like CAR-T, where each dose is manufactured from an individual’s own cells, the economics are even more strained. Disparities in access extend beyond price to infrastructure and regulatory capacity. Many countries lack the specialized medical centers, cold-chain logistics, or regulatory frameworks needed to deliver these therapies safely.²⁴ Biosimilars help by introducing competition once patents expire, but the complex approval pathway keeps their prices higher than traditional generics. The gap between what biotherapy can do and who can actually get it remains one of the field’s defining tensions.

Emerging Approaches

Several next-generation biotherapies are moving from lab bench to clinic. One of the most promising is CAR-NK cell therapy. Natural killer cells can be engineered with chimeric antigen receptors just as T cells can, but they carry inherent advantages. NK cells have a safer side-effect profile, can be manufactured from donor cells rather than requiring each patient’s own blood, and possess innate tumor-killing ability even without the engineered receptor.²⁵ They can be expanded from blood, cord blood, or stem cells and even cryopreserved for off-the-shelf availability, which would dramatically reduce wait times and manufacturing costs compared to current CAR-T protocols.²⁶ Researchers are also exploring CAR-NK cells against solid tumors, an area where CAR-T therapy has struggled.²⁷

On the delivery side, lipid nanoparticles, the same technology that carried mRNA in COVID-19 vaccines, are being adapted to transport therapeutic antibodies directly inside cells. Antibodies have traditionally been limited to targets on the outside of cells because they cannot cross cell membranes on their own. New LNP formulations can ferry full-length antibodies into the cytosol, where they can inhibit disease-driving proteins that were previously unreachable. Early work has demonstrated this in cancer cell lines and in animal models of Parkinson’s disease and acute lung injury.²⁸ If this pans out, it could vastly expand the range of diseases treatable with antibody-based biotherapy.

Phage Therapy and Living Biotherapeutics

Biotherapy is not limited to cancer and autoimmune disease. One of the more unusual branches involves bacteriophages, viruses that naturally infect and kill bacteria. Phage therapy predates antibiotics and is now experiencing a renaissance as antibiotic resistance worsens worldwide. Phages are highly specific, targeting particular bacterial species while leaving the rest of the body’s microbial ecosystem largely intact. They can also self-amplify at the site of infection and disrupt the protective biofilms that make some infections so stubborn to treat.²⁹

Recent developments have pushed the field well beyond simply dosing patients with wild-type phages. Genetic engineering tools, including CRISPR-based systems, are now being used to create phages with enhanced capabilities, and successful compassionate-use programs have built real-world clinical experience. Combination strategies pairing engineered phages with conventional antibiotics are also gaining traction.³⁰ Unlike most other biotherapies, phages can co-evolve with their bacterial targets, giving them a dynamic quality that static drugs lack. The regulatory path for phage therapy remains less established than for antibodies or cell therapies, but the urgency of drug-resistant infections is pushing agencies to develop new frameworks.