It is estimated that at least one-fifth of the world's population suffers from allergic diseases (atopic asthma and allergic rhinitis are the most common). Different factors contribute to the development of allergies, a predisposing genetic background is needed; however, environmental factors play an important role, among them exposure to infectious agents.

Allergy is increasing in the last decades particularly in Western industrialised countries, and according to the so-called hygiene hypothesis, this might be ascribable to the lower exposure to infectious agents, including parasitic helminths. Reasons for such a decrease are many: small family size, stability of intestinal microflora, affluent urban homes, large use of antibiotics, good sanitation which means low oral-faecal burden, and finally low or absent helminth infections. On the contrary, the low prevalence of atopy in developing countries is associated with high exposure to parasites in these regions.

The hygiene hypothesis has been revisited pointing out, for example, the role of IL-10 producing Treg cells, which are induced during chronic infections, in downregulating allergic responses. To verify the above hypotheses, a number of publications have appeared studying the effects of parasitic infections or of molecules derived from parasites on well-established experimental models of allergy, along with epidemiological studies, which have tried to correlate these conditions.

The aim of this special issue is to collect information on the relationship between allergic manifestations and parasitic infections from groups who have worked in this field in recent years, either as original research papers or reviews. Potential topics include, but are not limited to:

• Experimental models of allergy and parasitic infections
• Study of immunomodulating molecules derived from parasites
• Epidemiological studies on allergy and parasitic infections
• Clinical situations of allergy induced by parasitic infections
• Elucidating the role of host factors in the development of allergy

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