Even simple unicellular organisms such as bacteria possess enzyme systems that protect against viral infections. Basic immune mechanisms appeared in ancient life forms and remain in their modern descendants, such as plants and insects. These mechanisms include antimicrobial peptides (called defensins), phagocytosis, and the complement system. These make up the innate immune system, which defends the host from infections in a non-specific way. The simplest innate system is the cell wall or barrier on the outside to stop intruders getting in. For example, skin stops most outside bacteria getting in.