Innate and intrinsic immunity in planarians

Planarians in the Phylum Platyhelminthes occupy a peculiar phylogenetic position and have a strong regenerative capacity of their adult tissues, which has aroused general attention. Planarians are spontaneously exposed to various pathogens (microbes and harmful chemicals), but typically survive these challenges. Therefore, these animals can provide useful insights into the evolution of the innate immune system. This review mainly focuses on immune tissues (epidermis, pharynx, and intestine), immune cells (phagocytic reticular cells) and immune genes of planarians. In addition, we provide an overview of the critical proteins in the innate immune system - for example, pattern recognition receptors, complement system proteins, anti-microbial peptides and antioxidant enzymes. In particular, the effectors of the signaling pathways activated upon planarian infection are reviewed.