The Host-Parasite Interface I

Abstract

The functional interface between parasite and host is the site of molecular exchanges between the organisms: nutrients from the host enter the parasite and various products of the parasite metabolism enter the host. This is where the action is. It is also the place where the host can react against the parasite. Detailed studies of this interface have been done with only a relatively few host-parasite associations and we are only beginning to learn of the many complex interactions that take place. The structural nature of the interface varies widely with the type of parasite and the site it occupies in the host. For a parasite like a hookworm with a relatively impermeable cuticle and a complete alimentary tract into which blood and other host materials are ingested, the functional interface involves both the alimentary tract lining and the external surface of the worm. For a cestode, with no alimentary tract, the interface is the surface of the worm; for a protozoan cell, it is the plasma membrane and the lysosomal membrane system. For intracellular parasites, additional membranes derived in part from the host cell are involved. Intimately related to the host-parasite interface are structures used by parasites to maintain an appropriate favorable position: hooks, suckers, attachment mechanisms of different kinds, including even modifications of the host cell induced by certain intracellular parasites. I will consider in detail both the structure and what is known about the function of certain selected host-parasite interfaces, proceeding from parasites of the alimentary tract of vertebrates to those of blood and tissue spaces and finally to intracellular parasites.