The concept that microbial exposure, particularly to gastrointestinal flora, is a key element in promoting normal postnatal maturation of immune competence has been well established in the literature for many years, in particular relating to the rebalancing of Th1/Th2 immunity to redress the Th2 imbalance that is a feature of healthy immune function

in the fetus [15]. However, it has become increasingly clear that Th cell function represents only the ‘tip of the CDK inhibitor iceberg’ in this context, and that immune mechanisms across the full spectrum of innate, adaptive T effector and T regulatory functions are variably compromised in early life [16–19]. Moreover, this general principle that immune function undergoes major maturational changes during the early postnatal period has implications beyond allergic disease pathogenesis. The most obvious example involves responses to vaccines administered during early infancy. this website This issue is discussed in more detail in another section of the workshop

[20], but briefly, the intrinsic Th2 polarity of the infant immune system sets the stage for equivalently polarized initial T cell responses to vaccines, at least to those such as DtaP (diphtheria, tetanus and pertussis), which lack any Toll-like receptor (TLR)-stimulatory components [21]. This is not seen with vaccines such as bacillus Calmette–Guérin (BCG), which contain strong Th1-stimulatory components [22], and indeed the inclusion of a single dose of the DTwP vaccine in the initial three-dose priming schedule appears enough to ‘balance’ the ensuing Th memory response [23]. Of note, this tendency mafosfamide towards excessive Th2 polarity in memory responses to DTaP is strongest among children with a positive family history of atopy (AFH+) in whom Th1 function is most attenuated during infancy, and who represent the high-risk group with respect to

subsequent development of allergy and asthma. Additionally, slow postnatal development of Th1 function is associated with increased risk for early respiratory infection with viruses such as respiratory syncitial virus (RSV), as demonstrated in independent cohort studies in Australia [21] and the United States [24]. The notion that microbial exposure during early life might drive the postnatal maturation of immune competence and hence protect against atopy has been discussed widely over the last 15 years, and is supported in principle by a wide body of experimental animal data [25]. In particular, the role of the commensal flora in the gastrointestinal tract (GIT) appears to be of paramount importance [26], and it is generally accepted that signals from these organisms mediate the progressive transition from the fetal (Th2 polarized) to the more balanced immunophenotype observed in healthy children beyond infancy.