Up: Photo of Planaria Torva - http://www.biopix.com/planaria-torva_photo-49901.aspx
Not much is known about the immune system of the Planaria Torva, but what is known shows that its immune system relates to it's greatest feature: regeneration. Planaria worms are known to have approximately 30% of their body cells are stem cells, meaning they can regenerate any body part, including the head. The worm has an interesting immune system as if any part of its body is infected by a pathogen, it can release that body part and regrow it, taking care of the problem quickly.