Incidence of allergic diseases such as asthma has increased at an alarming rate in Western countries in the past few decades. However, in parts of the world in which parasitic nematode infections are highly prevalent, allergy remains uncommon. Hence, it has been postulated that nematodes offer humans protection against this type of disease. This article reviews the evidence to support this idea, considering data from human studies and results from investigations into the protective effects of nematodes in animal models of allergic disease. The evidence strongly favors a protective role for nematodes; thus, the search is on to find the molecules involved, with a view toward using them for therapeutic purposes. The article also describes the nature and mode of action of recently characterized nematode-derived molecules with antiallergic properties and highlights their therapeutic efficacy in allergy models.