It is estimated that approximately 20% of the world’s population suffer from allergic diseases. As the rates continue to increase, scientists are digging deeper into the different factors that contribute to the cause of allergies. This has brought forth a curious culprit – parasites, and in particular, helminths. Although infectious agents, including parasites, are known to play an important role in the development of allergies, it remains unclear whether this association is harmful or beneficial.
In general, allergies or allergic diseases, result from the improper targeting of the immune system against otherwise harmless environmental substances, such as peanuts or pollen. Substances capable of inducing this reaction are referred to as allergens. Depending on the person, specific allergen, and mode of introduction, the effects of allergies can range from mildly uncomfortable to life threatening anaphylaxis.
The first encounter of an allergen by the immune system results in Th2 immunity. These Th2 cells, a subset of T cells, interact and communicate with B cells, which become stimulated to produce IgE antibodies. Secreted IgE circulates in the blood and binds to an IgE-specific receptor on the surface of other immune cells, including mast cells and basophils. It is at this point that the IgE-coated cells become sensitized to the allergen. If a second encounter occurs, the allergen can bind to the specific cell surface IgE molecules and trigger a process resulting in the release of chemical mediators including histamine. This leads to the associated symptoms of allergies, such as rhinorrhea and rashes.
Since IgE also fights off parasitic infections, some scientists argue that the Th2-IgE mediated immune mechanism first originated to defend humans and animals against parasites. However, due to increased sanitation, parasite infections have been declining in recent decades. Consequently, this immune mechanism has begun targeting other molecules, resulting in the occurrence of allergic diseases. To support this notion, computational biologists from the EMBL-European Bioinformatics Institute in the United Kingdom identified 2445 parasite-derived proteins that demonstrate significant similarities to allergen-derived proteins. Of special interest, SmBv1L, a protein from the helminth Schistosoma mansoni resembles an allergenic protein in birch pollen. The same scientists then collected blood from 222 people in Uganda harbouring a S. mansoni infection and found that 1 in 6 people produced IgE antibodies that bound to SmBv1L. This is the first example of a plant pollen-like protein in a helminth that is also a target of IgE, suggesting that allergies could be a helminth immune response gone rogue.
In contrast, other scientists argue that helminth infections could in fact counteract allergic diseases. Helminths and similar parasites used to be widespread until sanitation became common practice. Several common parasites, including helminths, secrete chemicals into the gut post-infection that suppress the immune response, inhibiting an anti-parasite attack. Thus, some scientists contend that co-evolution of humans and parasites led to an immune system that functions correctly only in the presence of parasites. Similarly, the hygiene hypothesis maintains that increased health standards and sterile conditions expose children to fewer infections or microbial agents, and as a result, there is a failure to acquire appropriate immune regulation leading to unbalanced or misguided responses. In support of this idea, multiple epidemiological studies demonstrate an inverse relationship between asthma and helminth infections. M.C.F. Almeida and colleagues found increased asthma severity after repeated anti-helminth treatments. Furthermore, L.S. Cardoso and colleagues showed a negative correlation between infection with the helminth Ascaris lumbricoides and asthma. These studies suggest a potential benefit of helminths on allergic diseases.
Helminth infections have been associated with both a causal role and decreased severity of allergies. Further research is needed to better understand the function of helminths and parasites on allergic disease pathogenesis – are helminths part of the cause, or the solution? As for now, it appears they are a part of both.