Type 2 innate immune responses and the natural helper cell.

The T helper type 2 (Th2) immune response, characterized by the production of interleukin-4 (IL-4), IL-5 and IL-13, is a critical immune response against helminths invading cutaneous or mucosal sites. It also plays a critical role in the pathophysiology of allergic diseases such as asthma and allergic diarrhoea. The Th2 cytokines are induced soon after helminth infection, even before a pathogen-specific adaptive immune response is established. Recent studies have shed light on such innate Th2 cytokine production by formerly uncharacterized innate immune cells such as natural helper cells capable of producing Th2 cytokines in response to IL-25 and IL-33 independently of adaptive immune responses. These cells produce large amounts of Th2 cytokines, most notably IL-5 and IL-13, leading to eosinophilia and goblet cell hyperplasia. We discuss here the mechanisms of innate production of Th2 cytokines in host immune responses against helminth infection as well as allergic immune responses and the similarities and differences between recently identified Th2-cytokine producing cells.