How Do Immune Checkpoint Inhibitors Work? (as of 2026)

Immune checkpoint inhibitors are not drugs that kill cancer directly—they block signals like PD-1/PD-L1 so that your own immune cells can recognize and attack cancer cells again. They don't work for everyone; eligibility is determined by test results such as PD-L1 expression or MSI status.

Key Takeaways

• Immune checkpoint inhibitors work by blocking the signal between PD-1, a brake on T cells, and PD-L1 on the surface of cancer cells, helping immune cells attack the cancer (National Cancer Center Korea).
• Who is eligible is determined by markers such as PD-L1 expression level, MSI-High or MMR deficiency, and tumor mutational burden, and the relevant markers differ by cancer type.
• Depending on the drug, infusions are given every 2, 3, or 6 weeks, with each session lasting roughly 30 minutes to an hour.
• Response is typically assessed by CT every 2–3 months, and if the treatment is working and side effects are tolerable, it is generally continued anywhere from several months up to 2 years.
• Because these drugs cause the immune system to attack normal organs as a distinctive side effect, blood tests and thyroid function checks are repeated before every cycle.

What does it mean to "release the brakes"?

• T cells carry built-in brakes to prevent excessive attacks, and PD-1 is one of those switches. Some cancer cells display large amounts of a signal called PD-L1 on their surface, pressing down on this brake so that immune cells pass right by the cancer without attacking it.
• Immune checkpoint inhibitors block the connection between this switch and the signal. Once the brake is released, the immune cells that were already there begin attacking the cancer. Because the drug doesn't kill cancer directly but instead mobilizes the immune system, the speed of response and the nature of side effects differ from conventional cytotoxic chemotherapy.
• Response can sometimes appear late and continue even after treatment ends, but on the other hand, side effects can occur where the immune system attacks normal organs as well.

Which cancers and stages is it used for?

• It is most widely used for advanced or metastatic cancers that are difficult to operate on. It is used alone or combined with chemotherapy in lung cancer, gastric cancer, liver cancer, kidney cancer, bladder cancer, head and neck cancer, melanoma, and some colorectal and esophageal cancers. More recently, it has also been considered as perioperative adjuvant therapy aimed at lowering the risk of recurrence.
• Tumors confirmed to have MSI-High or MMR deficiency tend to respond well to immunotherapy regardless of the primary site, which is why this test is important across many cancer types. Conversely, if these markers are low and immune cells have difficulty infiltrating the tumor, the drugs tend to be less effective.
• Patients with autoimmune disease, those on immunosuppressants following organ transplant, or those on high-dose steroids require careful judgment before treatment.

What is the process from testing to treatment?

• Step 1 — Tissue samples are tested for markers such as PD-L1, MSI/MMR. If the tissue sample is old or insufficient, a new sample may need to be obtained.
• Step 2 — Once the drug is selected, it is given by IV infusion, repeated every 2, 3, or 6 weeks depending on the regimen.
• Step 3 — Because the first infusion requires observation for reactions, admission or a day of monitoring is recommended.
• Step 4 — Response is then typically assessed by CT every 2–3 months; if the drug is working and side effects are tolerable, treatment continues anywhere from several months to 2 years.
• Step 5 — When to stop treatment is decided by the treating medical team based on cancer type, response, and side effects. A stay in Korea of 2–3 weeks is common when including testing and the first cycle, after which subsequent cycles can continue through regular visits or by handing off the treatment plan to local physicians.

FAQ — How do side effects differ from chemotherapy?

The most common side effects are fatigue, itching, rash, and reduced appetite, but the side effects unique to immunotherapy arise from the immune system attacking normal organs. These can include hypothyroidism or hyperthyroidism, diarrhea from colitis, elevated liver enzymes, pneumonitis, skin reactions, and, rarely, adrenal or pituitary insufficiency or diabetes. It has also been reported that hormone-related problems are often permanent and may not reverse even after stopping treatment (U.S. National Cancer Institute [NCI], NCCN patient guidelines). These effects can appear not only weeks after starting treatment but also after treatment ends, so new symptoms should be watched for over at least 1 year.

FAQ — Is this the same as treatments that culture and inject cells?

No, it is different. The immunotherapy discussed here refers to immune checkpoint inhibitors that are included in international treatment guidelines. Among cell-culture-and-infusion type treatments, only a limited number are recognized as standard care, and whether there is evidence supporting your particular case can be confirmed during the remote review (pre-arrival review) stage based on your test results.

Sources and Notes

• This article was written based on materials from the National Cancer Center Korea, NCCN patient guidelines, and the U.S. National Cancer Institute (NCI).
• This content is intended for general medical information purposes only, and diagnosis, treatment methods, and outcomes may vary depending on individual condition. Accurate diagnosis and treatment should always be determined through consultation with a medical professional.