A healthy immune system is so well-organized, so effective against invasion, that its owner can be forgiven for never giving it a second thought. Nevertheless it is a highly complex dimension of human physiology, and when it is overwhelmed, healing is no easy task. This program explores the three principal levels of the immune system—what keeps them functioning in top form and what duties they carry out for the human body. Overviewing the most visible weapons in the first line of immunological defense (skin, sweat, tears, and other physical mechanisms), the video illustrates protein-based responses, cell-based responses, inflammation, clotting, fever, and humoral immunity. Eye-catching animation helps clarify anatomical and biochemical concepts. Viewable/printable educational resources are available online. (21 minutes)