What Is Immunotherapy? How PD-1/PD-L1 Inhibitors Work, Which Cancers They Treat, and Side Effects

Immunotherapy is a treatment approach that fights cancer cells by activating the body's own immune system. It does not kill cancer cells directly; instead, it "removes the seal that cancer cells place on the immune system," allowing the T cells responsible for attacking enemies to recognize and attack the tumor again.

Since the arrival of immune checkpoint inhibitors, immunotherapy has rewritten treatment standards for more than a dozen cancers, including lung cancer, melanoma, liver cancer, and kidney cancer. Some patients with advanced disease have even achieved long-term remission lasting years—results that were hard to imagine in the chemotherapy era.

How It Works

The immune system can normally recognize and clear abnormal cells, but cancer cells are very cunning: they use molecules such as PD-L1 to bind to PD-1 on the surface of T cells, sending the immune system a false signal that says "I'm one of you, don't attack," as if wearing an invisibility cloak.

Immune checkpoint inhibitors (such as PD-1, PD-L1, and CTLA-4 inhibitors) work by tearing off that cloak: after the drug blocks the binding of PD-1/PD-L1, T cells recognize cancer cells again and launch an attack. Immunotherapy also includes other forms, such as CAR-T cell therapy (engineering a patient's own T cells and giving them back, used for some blood cancers) and cancer vaccines, among other cutting-edge directions.

When It Is Used

The range of uses for immunotherapy keeps expanding: advanced non-small cell lung cancer (used alone based on PD-L1 expression or combined with chemotherapy), melanoma, liver cancer (combined with targeted therapy), kidney cancer, head and neck cancer, esophageal cancer, bladder cancer, triple-negative breast cancer, and Hodgkin lymphoma all now have established roles for immunotherapy. Tumors with MSI-H/dMMR features usually respond particularly well to immunotherapy, regardless of where the cancer started.

In recent years, immunotherapy has also moved from advanced disease to earlier stages: preoperative (neoadjuvant) immunotherapy can shrink tumors and improve the chance of a cure; postoperative adjuvant immunotherapy lowers the risk of recurrence. Whether immunotherapy is suitable depends on PD-L1 expression, MSI/TMB test results, and the cancer type, and should be assessed comprehensively by a doctor.

Common Side Effects and Management

The side effects of immunotherapy are completely different from those of chemotherapy: the activated immune system sometimes "mistakenly attacks" normal organs, which is called immune-related adverse events. Common ones include skin rash, diarrhea (colitis), hepatitis, and thyroid abnormalities; less common but serious ones include pneumonitis, myocarditis, and hypophysitis.

The key is early detection: during treatment, blood tests are done regularly to monitor liver and thyroid function, among others; if persistent diarrhea, unusual fatigue, shortness of breath, or worsening rash occurs, tell your medical team immediately. Most immune-related side effects can be effectively controlled with steroids, and early management usually does not affect long-term treatment. Immune side effects can appear at any time during the treatment course, even months after stopping the drug, so vigilance must continue.

Living Well During Treatment

Immunotherapy is usually given as an intravenous infusion every two to six weeks, taking about half an hour to an hour, and its impact on daily life is usually smaller than that of chemotherapy. Maintaining a balanced diet, regular exercise, and adequate sleep helps the immune system perform at its best.

Two special reminders: First, do not take Chinese herbal medicines or supplements claimed to "boost immunity" on your own, as they may interfere with treatment; ask your doctor before using any supplement. Second, carry a card stating which immunotherapy drug you are receiving, so that if you seek care for side effects, emergency doctors can immediately understand your situation.

Clinical Trials: One More Treatment Option

Immunotherapy is currently the most active area of cancer research: new checkpoint targets (such as LAG-3 and TIGIT), bispecific antibodies, CAR-T and TIL cell therapies, and personalized cancer vaccines are all advancing rapidly in clinical trials.

The CancerLink platform can screen for immunotherapy-related clinical studies recruiting in Hong Kong based on cancer type, stage, and genetic mutations. After successful matching and enrollment, study-related treatment costs are fully covered. For patients who respond poorly to standard treatment, trials are an important way to access cutting-edge therapies.

Immunotherapy is making "living with cancer" a reality for more and more patients with advanced disease. Whether it is suitable depends on the cancer type and biomarker testing—bringing this question to your oncologist may open a new door.