“There’s 40 million T cells in this little thing,” graduate student Alex Salter said as he held up a tightly sealed, plastic flask about the size and shape of a pocket paperback. The liquid inside sloshed gently as he stepped aside to let one of his colleagues in the busy lab at Fred Hutchinson Cancer Research Center go by. Chock-full of nutrients, the orange Kool-Aid-colored growth medium in the flask was clouded by the hordes of human cells that had been happily multiplying there for days.

These cells were Salter’s babies. He had genetically engineered their forebears to carry different versions of a cancer-targeting molecular weapon, and he was readying to put his burgeoning T-cell army to the test in dishes full of breast cancer cells. He wanted to know: Which ones of these kill cancer the best, and why?

A type of immune cell, T cells are at the heart of a class of emerging therapies that seek to aim the power of the immune system at cancer. Two approved therapies for certain advanced blood cancers involve genetically reprogramming patients’ T cells to target their tumors, and many more such strategies are in the pipeline, including at Fred Hutch. These therapies and others still in development have shown promise in eradicating even treatment-resistant cancers that have penetrated throughout patients’ bodies.

But today’s T-cell therapies are like the Wright Flyer was to aviation. While many problems have been solved to get to this point, there are so many innovations yet to come. And that’s why T-cell tinkerers like Salter and his colleagues are hard at work.

The molecular weapons Salter is working with are artificial receptors, called chimeric antigen receptors, or CARs. Embedded in T cells, CARs and their natural cousins, T-cell receptors, or TCRs, are the tools that allow T cells to recognize cancer and trigger an immune attack.

T cells are only about as wide as a strand of spider silk. So it can be hard to fathom the teensy scale of TCRs and CARs, the finely constructed molecular machines that stud their surface. Salter is a member of the lab of Dr. Stanley Riddell at Fred Hutch, whose research group is one of many now learning how tiny variations in the complex structures of these receptors can have a big impact on patients: namely, whether an engineered T-cell product eliminates a cancer and, crucially, how durably and safely it does so.

“Some of the design problems have been able to be solved,” Riddell said, citing a variety of improvements that scientists have made to the cancer-recognizing parts of CARs. But, he added: “There’s still many, many questions.”

Scientists around the world are working to save more lives by tinkering with T cells and their precision-engineered, cancer-fighting receptors.