A fairly good knowledge base concerning various steps in allergic reactions now exists, but despite this knowledge the prevalence of allergic diseases is still increasing: in some areas of the industrialised world up to 50% of the population is affected. Juan Gispert at Quintiles provides the first paper in a two part series, describing the pathological role of immunoglobulin E (IgE) in allergic reactions, and how identification of this role has allowed physicians to treat reactions by addressing the underlying immunological mechanisms. The second paper in the series will discuss unique challenges for clinical trials in this therapeutic area.