The immunohomeostatic role of hydatid cyst toxins (Echinococcus granulosus)

It is obvious that during the evolution of a host-parasite relationship, in which one animal is needing the other, while the latter is trying to expel or destroy the former, both partners tend to reach an equilibrium. In many cases they are still far off, but in hydatidosis (Echinococcus granulosus) it would appear that a lot has been achieved. As the near equilibrium is mainly related to the state of the immunological relationship, it is in fact a case of immunohomeostasis. The immunological reactions of the host involve both humoral and cellular phenomena. The nature of the humoral response varies greatly according to host species, parasite strain and several factors of the infection and analysis procedures. The cell-mediated defense mechanism involves various types of cells, but activated macrophages appear to be most important. With regards to the modes of immunoevasion, attention is paid to the barrier role of the laminated layer, the inactivation of complement and the interference with the activities of host immune cells. The role of toxic substances released by protoscoleces and cysts is especially emphasized. The toxins appear extremely vulnerable to host macrophages and play a very important role in the establishment of a hydatid cyst. However, they have only a local influence and do not jeopardize the development of concomitant immunity. Observations in experimental secondary echinococcosis led to the conclusion that the establishment of a hydatid cyst depends on the speed of the parasite's toxic effect versus the host immunological reaction. Survival of the parasite is based on the continued balancing effects of both.