This animation shows the immune system reacting (or overreacting) at the cellular level. It describes what happens when an allergen, like dander or pollen, enters the body. It also shows how some allergy medicines work, like anti-histamines.

The animation explains how the lungs provide oxygen to and remove carbon dioxide from the blood. The specialized anatomy of the air sac in the lung allows the gas exchange to occur efficiently, as discussed and shown in the video.