This animation shows how a cell infected by a virus signals cytotoxic T cells to destroy itself.

In order to prevent a virus from spreading, cytotoxic T cells (also known as CTLs) identify and eliminate cells that have already been infected. The animation illustrates how infected cells cut up viral proteins and present the fragments, called antigens, on their surfaces using MHC class I molecules. When a cytotoxic T cell recognizes that the protein fragment is foreign in origin, it triggers the release of proteins that destroy the infected cell, preventing it from producing more viruses.

This animation is a clip from a 2007 Holiday Lecture Series, AIDS: Evolution of an Epidemic. Depending on students’ background, it may be helpful to pause the animation at various points to discuss different proteins or structures.