Immunotherapy is a cancer treatment that works by strengthening or retraining your own immune system to find and destroy cancer cells. Unlike chemotherapy or radiation, which directly kill cancer cells (along with healthy ones), immunotherapy targets the immune system itself, helping it recognize threats it was previously missing. It’s now approved for dozens of cancer types and has dramatically improved survival for some of the hardest-to-treat cancers.
Why Your Immune System Misses Cancer
Your immune system is built to detect and eliminate abnormal cells, and it does this constantly. But cancer cells develop ways to hide. Some disguise themselves as normal tissue. Others produce proteins on their surface that act like an “off switch” for immune cells, telling them to stand down. Immunotherapy works by disrupting these evasion tactics so your immune cells can do the job they were designed for.
Types of Immunotherapy
There are several distinct approaches, each working through a different mechanism. Your oncologist may recommend one type or a combination depending on your cancer.
Checkpoint Inhibitors
These are the most widely used form of immunotherapy. Cancer cells often exploit natural “checkpoint” proteins on immune cells, proteins that normally prevent the immune system from attacking healthy tissue. By binding to these checkpoints, cancer essentially puts the brakes on your immune response. Checkpoint inhibitors release those brakes.
The main targets are proteins called PD-1, PD-L1, and CTLA-4. Drugs blocking these proteins are now approved for cancers including melanoma, lung cancer, bladder cancer, kidney cancer, Hodgkin lymphoma, ovarian cancer, and multiple myeloma, among others. The FDA continues to expand approvals, and newer drugs now target additional checkpoint proteins like LAG-3.
CAR T-Cell Therapy
This is a more personalized approach. Doctors draw your blood and extract white blood cells, then send those cells to a lab where scientists genetically modify your T-cells (a type of immune cell) by adding a lab-made gene. This gene gives the T-cells a new receptor that helps them lock onto cancer cells. The modified cells are then grown in large numbers over three to six weeks until there are enough to mount an effective attack.
Before receiving the new cells back, you undergo a short course of chemotherapy to prepare your body to accept them. The actual infusion takes only five to 30 minutes. CAR T-cell therapy has been particularly effective for certain blood cancers like leukemia and lymphoma that haven’t responded to other treatments.
Cancer Treatment Vaccines
These are not the same as preventive vaccines like the HPV vaccine. Treatment vaccines are given to people who already have cancer, and they work by training the immune system to attack cancer cells more aggressively. One approved vaccine uses a patient’s own immune cells (called dendritic cells) that are modified in a lab and reinfused to fight advanced prostate cancer. Another approach uses a modified virus that’s injected directly into tumors, where it replicates inside cancer cells and triggers an immune response.
How Doctors Decide if You’re a Candidate
Not every cancer responds to immunotherapy, and not every patient with a responsive cancer type will benefit equally. Doctors use several biomarkers, measurable characteristics of your tumor, to predict how likely you are to respond.
The most established test measures how much PD-L1 protein your tumor produces. In clinical trials, patients whose tumors tested positive for PD-L1 had a response rate of about 48%, compared to 15% for those whose tumors tested negative. That 15% is important: some patients respond even without the expected biomarker, which is why testing is a guide rather than a guarantee.
Doctors also look at tumor mutational burden, essentially how many genetic mutations your cancer carries. Tumors with more mutations tend to produce more abnormal proteins that the immune system can target, making immunotherapy more effective. A third key marker is microsatellite instability, a sign that a tumor’s DNA repair machinery is broken. Cancers with this feature respond well enough that the FDA approved a checkpoint inhibitor for any MSI-high cancer regardless of where in the body it originated, a first-of-its-kind approval.
Certain genetic mutations can also signal that immunotherapy is unlikely to work. For example, lung cancers carrying a specific mutation called STK11 respond to checkpoint inhibitors only about 7% of the time, compared to roughly 29% for lung cancers without it.
What Treatment Looks Like Day to Day
Most immunotherapy is given as an intravenous infusion, though some newer formulations are injected under the skin. Treatment schedules vary widely. Some regimens are weekly, others every two to four weeks. Like chemotherapy, immunotherapy is typically given in cycles with rest periods in between, allowing your body time to recover and respond.
How long you stay on treatment depends on your cancer type, how you respond, and how well you tolerate the side effects. Some patients receive immunotherapy for a defined period (often one to two years), while others continue as long as it’s working.
Combining Immunotherapy With Other Treatments
Immunotherapy is frequently paired with chemotherapy, radiation, or other immunotherapy drugs. This isn’t just throwing more treatments at the problem. Low-dose chemotherapy can actually make tumors more visible to the immune system by causing cancer cells to release signals that attract immune cells. Combining immunotherapy with certain chemotherapy drugs has shown increased effectiveness compared to either treatment alone. Some of the most significant recent FDA approvals have been for combination regimens rather than single drugs.
Side Effects and Why They Happen
Because immunotherapy revs up your immune system, the main risk is that it attacks healthy tissue along with cancer. These are called immune-related adverse events, and they can affect virtually any organ. The side effects are fundamentally different from chemotherapy’s. Instead of nausea from cell damage, you’re dealing with inflammation from an overactive immune response.
Skin reactions are the most common, affecting up to 72% of patients. These range from rashes and itching to vitiligo (loss of skin pigment). Digestive issues come next: diarrhea occurs in up to 54% of patients on certain regimens, particularly those combining two checkpoint inhibitors. Joint and muscle pain affects up to 40% of patients in clinical trials.
Less common but more serious effects include inflammation of the lungs (around 3% of patients on PD-1 inhibitors) and hormone disruptions from inflamed glands, which occur in roughly 10% of patients. About 10% of patients develop endocrine problems that may require long-term hormone replacement, such as thyroid medication. Most side effects are manageable when caught early, which is why your care team will monitor you closely with regular blood work and check-ins throughout treatment.
How Well It Works
The impact of immunotherapy has been most dramatic in advanced melanoma. Before checkpoint inhibitors became available, only about 10% of patients with advanced melanoma survived five years. With first-line immunotherapy, that number has jumped to roughly 36%, according to a large study published in JAMA Network Open analyzing outcomes from 2012 to 2019. That’s a nearly fourfold improvement in a cancer that was once considered almost untreatable at advanced stages.
Results vary significantly by cancer type. Some cancers, particularly those with high mutational burden or microsatellite instability, respond exceptionally well. Others show more modest improvements. And immunotherapy doesn’t work for everyone: even in the most responsive cancers, a significant percentage of patients see no benefit. The ongoing challenge is figuring out, before treatment starts, who will respond and who won’t, so patients can be matched to their most effective option as quickly as possible.