We are bombarded by biological threats from the outside, ranging from toxic particulates to epidemic viruses, and also by threats from within, like cancer. How do our bodies manage to be victorious against these threats for so many years, in most cases? Many people have some familiarity with aspects of the answers now, due to COVID-19. But how can these defense mechanisms actually work, and how can they coordinate their actions to be effective and safe? Why do they fail? This course will zoom between scales to introduce the cells that the body uses for immune defense and how they execute their roles, both system-wide and at the molecular level. A central theme will be how the system is controlled by cellular "software" reading the genetic code, by ultra-rapid evolutionary mechanisms, and by elegant cell-cell communication networks. Lectures and student presentations will be included. May be taken pass/fail if taken in a first-year student's first year. Limited enrollment.