How It Works: Cancer-Fighting Immunotherapy

Training the immune system to kill tumors

In the war against cancer, doctors have discovered a powerful new tool: the immune system. The FDA recently fast-tracked approval of three new immunotherapy drugs, called PD-1 inhibitors, designed to help white blood cells hunt down and eradicate hard-to-fight tumors–indefinitely. “Chemotherapy almost always stops working,” says Jonathan Cheng, executive director of oncology clinical development at Merck. “The promise of immune therapy is that you’re training the immune system to attack something foreign, so you’re able to maintain that activity for a very long time–hopefully for the rest of a patient’s life.”

How It Works

1. The Defenders

It’s the job of T cells to protect the body. When presented with bits of foreign protein from intruders, they activate and destroy infected cells.

2. The Checkpoint

To prevent an uncontrolled immune response, T cells carry a switch: a surface protein called the protein death 1 (PD-1) receptor. When engaged, it inhibits T cells’ proliferation.

3. The Inhibitors

Tumor cells overexpress proteins called programmed death ligands that fit into PD-1 like keys to a lock, essentially turning T cells off.

4. The Blockers

New drugs, such as Merck’s Keytruda, bind to PD-1 and block cancer cells’ access. The immune system unleashes additional T cells, which then find and attack tumors.

This article was originally published in the April 2015 issue of Popular Science, as part of our annual How It Works package.