# What is Checkpoint inhibitor?

Source: https://oncologyistanbul.com/glossary/checkpoint-inhibitor

> A class of immunotherapy drug that blocks the molecular "brakes" (checkpoints) tumors use to hide from the immune system.

Immune checkpoints are normal molecular signals that keep the immune system from attacking the body's own healthy tissue — some tumors exploit these same signals to avoid being attacked. Checkpoint-inhibitor drugs block that signal, releasing the brake so the immune system can recognize and act against the tumor. Whether a checkpoint inhibitor is likely to help depends on biomarker testing (such as PD-L1 expression or MSI-H/dMMR status), not the cancer type alone.

## Related
- [PD-L1 testing](https://oncologyistanbul.com/glossary/pd-l1-testing)
- [MSI-high / dMMR](https://oncologyistanbul.com/glossary/msi-dmmr)
- [Immunotherapy](https://oncologyistanbul.com/treatments/immunotherapy)

## Authoritative references
- [Wikipedia](https://en.wikipedia.org/wiki/Immune_checkpoint)
- [Wikidata](https://www.wikidata.org/wiki/Q21686041)

Full glossary: https://oncologyistanbul.com/glossary