Allergic diseases—such as asthma, rhinitis, and eczema—have been increasing worldwide over several decades, and the increase has largely been attributed to a Western lifestyle (the hygiene hypothesis). However, the exact causes of the increase are unknown. Several trigger mechanisms are probably involved, but it is still not clear whether similar triggers are responsible for the increases seen in different allergic diseases.1 In the linked systematic review (doi: 10.1136/bmj.b2433), van den Oord and Sheikh assess the association between defects of the filaggrin gene (FLG) and the risk of developing allergic sensitisation and allergic disorders.2

Allergies form a heterogeneous group of diseases in which IgE mediated reactions are the key feature, and a wide variety of symptoms can occur. Diagnosis is largely based on the history, symptoms, and detection of IgE …