Immune-mediated diseases and microbial exposure in early life.

The non-communicable disease pandemic includes immune-mediated diseases such as asthma and allergy, which are likely originating in early life where the immature immune system is prone to alterations caused by the exposome. The timing of exposure seems critical for the developing immune system, and certain exposures may have detrimental effects in the earliest life, but no or even beneficial effects later. The human microbiome and infections are candidates as intermediary in the interaction between the host and the environment. The evidence seems inconsistent as infections as well as particular colonization patterns in neonates drive both short-term and long-term asthma symptoms, while, on the other hand, the composition of the microbiome in early life may protect against asthma and allergy in later life. This apparent contradiction may be explained by a deeper disease heterogeneity than we are currently able to discriminate, and in particular, the indiscriminate lumping together of different diseases into one atopic disease category. Also, the microbiome needs a differentiated understanding, considering balance between microbial groups, diversity and microbial genetic capability. Furthermore, the effects of the microbial exposure may only affect individuals with certain susceptibility genes. Few of the observations have been replicated, and publication bias is likely. Therefore, we are still far from understanding, or having proved, causal effects of the human microbiome. Still, the microbiome-gene interaction is a fascinating paradigm that fosters exiting research and promises a breakthrough in the understanding of the mechanisms driving asthma, allergy and eczema, and potentially also other immune-mediated non-communicable diseases.