Biologists would like to train a patient’s own immune system to treat diseases such as cancer. German researchers have reported a new step in that direction with a method that traps immune system T cells inside microscopic emulsion droplets and then exposes the cells to chemical signals that could teach them to coordinate attacks on disease targets (J. Am. Chem. Soc., DOI: 10.1021/ja311588c). Previous methods for training cells relied on two-dimensional surfaces, but the new approach mimics the three-dimensional environment inside the body in which T cells learn about possible threats. A team led by Joachim P. Spatz of the Max Planck Institute for Intelligent Systems created an artificial 3-D environment by mixing two streams of liquid in a microfluidic system: a fluorinated polymer surfactant linked to gold nanoparticles dissolved in oil and an aqueous mixture of T cells. When the streams meet, water-in-oil droplets form with the nanoparticles on the water-facing surface and T cells enclosed inside. When the researchers decorated the gold particles with protein fragments known to interact with T cells, the cells latched on as if they were interacting with other immune cells. In the future, a doctor could isolate a cancer patient’s T cells, expose the cells to antigens specific to the cancer, and then transplant the cells back to direct the immune system to attack a tumor.