Purpose of review: Treatment and even prevention of allergic asthma will require a detailed understanding of disease pathogenesis and in particular identification of factors that govern T-helper type 2 (Th2) immunity. This review defines the priming and differentiation steps necessary to develop antiallergen Th2 immunity and highlights recently identified stimuli that satisfy these requirements.
Recent findings: Striking discoveries in innate immunity have advanced our understanding of how adaptive immune responses are initiated, yet only recently have these principles been applied to allergic disease. Signaling through certain innate immune receptors, the toll-like receptors (TLR) have been shown to modulate Th2-mediated disease in animal models. The dendritic cell has emerged as the central player in the intricate interplay between the adaptive and innate systems of immunity. Recent studies have also uncovered alternative pathways of initiating allergen sensitization that depend entirely on adaptive, rather than innate immune, triggers.
Summary: The adaptive immune system cannot initiate a response without the "permission" of the innate immune system, and this holds true for Th2 responses to aeroallergens, although induction of Th2 immunity in response to TLR signaling varies with the type and dose of TLR ligand. However, under conditions of ongoing Th2 inflammation, the adaptive immune system can act as its own adjuvant and provide the necessary activating signals to initiate an immune response to foreign protein antigens. This may be the mechanism underlying the clinically observed phenomenon of polysensitization in atopic patients and provides another therapeutic target in asthma.