Mini-review
Helminths and our immune system: Friend or foe?

https://doi.org/10.1016/j.parint.2009.02.001Get rights and content

Abstract

Helminths are able to modulate and suppress the hosts' immune response in order to promote their own survival. This ability to alter immune responses could be of potential detriment to the host if it interferes with the development of protective immune responses against other infections. However, helminth infections have also been suggested to be beneficial in the control of excessive inflammatory reactions. In this review I will highlight some of the data suggesting both positive and negative effects of helminth infections in humans and in experimental models.

Introduction

Helminth infections are amongst the most common infections of humans and are notoriously chronic in nature, with some species of worms surviving within their host for many years, in some cases even decades. In order to facilitate such longterm survival within an immunocompetent host these organisms have developed sophisticated survival strategies, including the ability to modulate and manipulate our immune system. This ability to alter immune responses could be of detriment to the host if it interferes with the development of protective immune responses against other infections. However, since helminths and the human immune system have co-evolved over millennia it has also been suggested that helminth infections may have a beneficial role in controlling autoimmune and other excessive inflammatory responses and that these organisms may even provide a source of novel anti-inflammatory therapeutics. Parasitic helminth infections may therefore constitute a double-edged sword for human health and it is of vital importance to clarify any beneficial or detrimental roles these infections may have on public health, in addition to their role as single infections.

Section snippets

Helminths and the immune system

The mammalian immune system is a carefully tuned orchestra consisting of a large number of different members, producing humoral (antibody) and cellular immunity. Central to most of the immune functions are T helper (Th) cells. Th cells play a critical part in instructing and controlling the various players of the immune response against invading pathogens. Several different types of T helper cells exist, primarily based on the type of cytokines they produce. Th1 cells secrete IFN-γ, are

Helminths and allergic disorders

Allergic diseases include asthma, atopic dermatitis and rhinitis. These are inflammatory diseases caused by immune reactivity to environmental allergens. The prevalence of allergic disease is generally low in rural areas of Europe [5] and in developing countries [6], while they are considered an increasingly important cause of morbidity in westernized countries. It is clear that children with several older siblings or attending day care have a reduced risk of developing allergies [7] and a

Helminths and vaccination

Many modern vaccines are designed to induce a Th1 response for optimal efficiency. Since helminth infections in general induce either a Th2 or a regulatory response, or both, there has been much discussion and speculation on whether concurrent helminth infections are responsible for reducing vaccine efficacy. Clear evidence exist from murine models that helminth infections can alter the nature of immune responses to non-parasite or ‘third-party’ antigens [67], [68], [69] via the skewing of the

Concluding remarks

There is now little doubt that helminth infections really do have the ability to influence the development of immune responses. Both animal and human studies have demonstrated this repeatedly and convincingly. However, what remains to be established is if this ability is of any real public health significance. It is clear that many studies are still conduced based on assumptions regarding what is probably a somewhat simplistic immunological view of helminths, who tend to be generally lumped

Acknowledgements

This work was supported by a Wellcome trust grant and a RCUK fellowship. I would like to thank Dr Quentin Bickle and Prof. Eleanor Riley for helpful discussions.

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