Scientists have a new tool to help them tease out the immune system's mysteries. Researchers cultured human B and T cells inside a microfluidic Organ Chip and coaxed them to form functional lymphoid follicles (LFs) -- structures that reside in lymph nodes and other parts of the human body and mediate immune responses. The LF Chip replicated human immune responses to both pathogens and a commercial influenza vaccine in vitro, offering significant improvement over existing preclinical models like cells in a dish and non-human primates.