Parasites are able to manipulate their host’s immune system in order to promote their own survival and replication. This evasion is necessary to allow them to cause a persistent and recurrent infection within their host. Furthermore, some studies suggested that this evasion is required for other parasitic functions, such as virulence that is the aptitude of the parasite to establish a productive infection. In order to achieve that, parasites are usually taking control of the infected cells and of the immune system in general. However, the precise mechanisms involved in this immune system subversion are very complex and vary between the different parasite species. These different mechanisms originated from the coevolution of human and parasites for thousands of years. Furthermore, several different mechanisms are usually used simultaneously during a parasitic infection. Here are some very simplified examples of this type of mechanisms susceptible to be used by parasites in order to evade their host’s immune system.
One of the mechanisms involved in the evasion of the host’s immune system consists in the selective activation or deactivation of certain cell types involved in the immune system. Actually, parasites are showing some specific antigens on their cell surface or secrete some enzymes that selectively activate or deactivate effective cells and components of the immune system. This mechanism allows the parasite to keep only the immune system’s cells that are harmless or needed for them and get rid of the immune response susceptible to target them. Actually, this mechanism helps to soften the general immune response.
Some parasites are also able to switch alternatively the different molecules expressed at their cell surface. This trick allows the parasite to evade the immune system. Actually, one type of surface molecule is expressed until some parasites begin to be killed by the immune system. At this point, a subpopulation of parasites switch to another surface molecules to avoid being targeted by the immune system, and then proliferate and contribute to the persistence of the infection.
Another evasion mechanism for the parasite consists to hide itself from the direct action of the immune system. For example, different species of protozoan parasites are able to infect one or many different cell types. While they are within these cells, parasites are hidden from the immune system cells and cannot be attacked by them. It is interesting to note that some parasites, such as Plasmodium\, must infect different cell types successively to fulfill their life cycle. Parasites are then very quick to switch from one cell type to another in order to avoid being detected by the effective immune system’s cells. Moreover, parasites can also hide themselves in some tissues that are known to be able to tolerate, to some extend, foreign antigens without eliciting an immune response, such as eyes and nervous system. They can also hide their eggs in fat body that are not usually targeted by the immune system’s cells.
Furthermore, some protozoan parasites are able to infect different immune system’s cells. For example, the protozoan parasite Leishmania needs to infect macrophages in order to fulfill its life cycle and have though developed many mechanisms to survive within these cells. Other parasites are also able to infect other immune system’s cell types. These parasites usually hijack the infected cells in order to promote their survival and replication. Then, they are able to modulate greatly the gene expression of the infected cells. As such, the infected immune system’s cells are expressing proteins helping the parasite to persist and evade the immune system.
Source: Centers for Disease Control and Prevention; Schmid-Hempel, P. (2009). Immune defence, parasite evasion strategies and their relevance for “macroscopic phenomena” such as virulence. Philosophical Transactions of the Royal Society B: Biological Sciences, 364(1513), 85–98.