Inside a target cell, an enzyme entering through perforin pores triggers programmed cell death. That enzyme is a:

1. Perforin punches the channel through the membrane. 2. A protein-digesting enzyme then follows it in. 3. That enzyme is a granzyme, and it triggers apoptosis. _Source: OpenStax Anatomy and Physiology 2e, Chapter 21, The Lymphatic and Immune System._