One of the factors limiting progress in this area is the inherent complexity of the flora, but the advent of genomics-based approaches is rapidly plugging this technology gap and the increasing application of this technology will hopefully clarify the role of the flora to

a significant degree in the near future. More indirect human data are available from the relevant epidemiology literature concerning the role of microbial pathogens as opposed to commensal flora [28,29]. The original conceptualization of the hygiene hypothesis envisaged infection frequency as the key factor discriminating high-risk and low-risk populations, but it has become evident that qualitative aspects of Selleckchem ABC294640 infection(s) may be of equivalent or even greater importance. In particular, there is strong evidence linking enteric infections with reduced risk for allergic sensitization

Decitabine nmr [30], and similarly for mild–moderate respiratory infections (without wheeze/fever) which spread to the lower respiratory tract [31], whereas upper respiratory tract infections do not appear to play such a role [32]. In contrast to the above, one class of viral infections has been linked epidemiologically in multiple studies to risk for subsequent development of asthma in childhood, notably moderate–severe viral infections which spread to the lower respiratory tract and which are of sufficient

intensity to trigger wheeze and/or febrile responses [32–34]. These infections serve as independent risk factors for subsequent asthma development, but their asthma-promoting effects are much stronger against a background or respiratory allergy (reviewed in [35,36]). On the basis of these findings, we have proposed a ‘two-hit’ model for asthma aetiology in early childhood (Fig. 1) in which interactions between inflammatory pathways involved in host responses to aeroallergens and viruses infecting the airway epithelium synergize to perturb early postnatal lung growth and differentiation, resulting in later expression of the asthma phenotype [35,36]. These ADAMTS5 interactions are most profound in children who are sensitized to aeroallergens early and who experience severe infections during the same period [32]. A key question remaining to be resolved fully relates to the nature of these interactions between the anti-viral and atopic pathways. We hypothesize that one important focus of these interactions is the network of airway mucosal dendritic cells (AMDC) first described in humans [37] and experimental animals [38,39] by our group, and which are now recognized generally to play an essential ‘gatekeeper’ role in control of immune responses in the respiratory tract to all classes of inhaled antigens and pathogens [40,41].