Your immune system constantly measures and protects you—not just from infections, but by maintaining a delicate balance with beneficial bacteria, monitoring mutations, and responding to your thoughts and emotions. Dr. Max Krummel, a leading immunologist at UCSF, explains how sleep deprivation impairs T cell migration, how memories can trigger immune responses, why the aging immune system becomes less effective, and why vaccine timing and spacing deserve scientific scrutiny alongside unwavering support for vaccination itself. T cells are individual sensors that continuously patrol your body measuring biomolecule concentrations; each of your 10^11 T cells can detect when something falls out of normal range and mount a response.