This edition of Science Screen Report illustrates how the human immune system protects against the pathogens humans encounter on a daily basis. It explains the function of first-line defenses and white blood cells, the distinction between viruses and bacteria, and the different functions of vaccines and penicillin. It also shows what happens when the immune system falters or fails, and how immunological understanding is increasing. Use this video to highlight a dimension of human anatomy that healthy people too often take for granted. A viewable/printable instructor's guide is available online. Correlates to National Science Education Standards. Produced in association with the Accreditation Board for Engineering and Technology and the Junior Engineering Technical Society. (17 minutes)