Résumé
This chapter discusses the mechanisms of invasion of a parasite into host tissue or host cells. Single cell protozoan parasites evade the host immune response by residing within host cells and enter the host cell with minimal trauma to ensure that they preserve an environment suitable for their replication and do not trigger a lethal host immune response. This intracellular invasion paradigm is also shared by a helminth parasite, Trichinella spiralis. Often an invasive larval form penetrates skin or the mucosa of the gastrointestinal tract to enter into the host.. The chapter states that the invasion phenomenon itself can be divided into three successive steps: recognition and attachment, internalization and vacuole development, vacuole maturation. Entamoeba histolytica, which is a protozoan, does not invade intra cellularly at any stage of its life cycle unlike other protozoas. The process of its invasion (extracellularly) and its properties are also elaborated in the chapter. .To complete the invasion process, helminth parasites have evolved sensory organs for finding and navigating within the host, attachment mechanisms utilizing specialized mouth structures or glue-like secretions, and hydrolytic enzymes to digest macromolecular barriers in the extracellular space