The effects of parasitic worms, or helminths, on the immune system is a recently emerging topic of study among immunologists and other biologists. Experiments have involved a wide range of parasites, diseases, and hosts. The effects on humans have been of special interest. The tendency of many parasitic worms to pacify the host's immune response allows them to mollify some diseases, while worsening others.
Immune response hypothesis
Mechanisms of immune regulation
Extensive research shows that parasitic worms have the ability to deactivate certain immune system cells, leading to a gentler immune response. Often, such a response is beneficial to both parasite and host, according to Graham Rook, a professor of medical microbiology at University College London. This immune "relaxation" is incorporated throughout the immune system, decreasing immune responses against harmless allergens, gut flora, and the body itself.
In the past, helminths were thought to simply suppress T-helper Type 1 (Th1) cells while inducing T-helper Type 2 (Th2) cells. Rook points out that this hypothesis would only explain the regulatory effects of parasitic worms on autoimmune diseases caused by Th1 cells. However, helminths also regulate Th2-caused diseases, such as allergy and asthma. Rook postulates that different parasitic worms suppress different Th types, but always in favor of regulatory T (Treg) cells.
Rook explains that these regulatory T cells release interleukins that fight inflammation. In the Journal of Biomedicine and Biotechnology, Osada et al. note that macrophages induced by Treg cells fight not only the parasitic disease, but also resist the immune system's response to allergens and the body. According to Hopkin, the author of a 2009 Parasite Immunology article on asthma and parasitic worms, other immunoregulatory mechanisms are also activated, including Mast cells, eosinophils, and cytokines that invoke a strong immunoglobulin E (IgE) response. All these fight a hyperactive immune response, reduce the inflammation throughout the body, and thus lead to less severe autoimmune diseases.
Osada et al. state that because parasitic worms may and often do consist of allergens themselves, the degree to which they pacify or agitate the immune response against allergens is a balance of their regulating effects and their allergenic components. Therefore, depending on both of these variables, some parasitic worms may worsen allergies.
In their Parasite Immunology article on worms and viral infections, Kamal et al. explain why some parasitic worms aggravate the immune response. Because parasitic worms often induce Th2 cells and lead to suppressed Th1 cells, problems arise when Th1 cells are needed. Such cases occur with viral diseases. Several examples of viral infections worsened by parasitic worms are described below.
Evolutionary theory
The positive effects of parasitic worms are theorized to be a result of millions of years of evolution, when humans and human ancestors would have been constantly inhabited by parasitic worms. In the journal EMBO Reports, Rook says that such helminths "are all either things that really do us no harm, or things where the immune system is forced to give in and avoid a fight because it's just a waste of time." In the journal Immunology, Rook states that, because parasitic worms were almost always present, the human immune system developed a way to treat them that didn't cause tissue damage.
The immune system extends this response to its treatments of self-antigens, softening reactions against allergens, the body, and digestive microorganisms. As the worms developed ways of triggering a beneficial immune response, humans came to rely on parasitic interaction to help regulate their immune systems. As developed countries advanced in technology, medicine, and sanitation, parasitic worms were mostly eradicated in those countries, according to Weinstock in the medical journal Gut. Because these events took place very recently on the evolutionary timeline and humans have progressed much faster technologically than genetically, the human immune system has not yet adapted to the absence of internal worms. This theory attempts to explain the rapid increase in allergies and asthma in the last century in the developed world, as well as the relative absence of autoimmune diseases in the developing world, where parasites are more common.