The role of skin-gut-lung microbiome in allergic diseases.

The incidence of allergic diseases has continued to rise in recent years, affecting ∼20% of the worldwide population especially children. Allergic diseases are chronic immune diseases that greatly reduce the quality of life of patients, leading to great economic and medical burden. The epidemiological studies indicated that children who had atopic dermatitis (AD) in infancy are more likely to develop food allergy (FA) later, and then allergic asthma (AA) and allergic rhinitis (AR) in childhood, which was defined as the "atopic march" (AM). Anatomically, AM follows a spatial sequence from the skin to the gastrointestinal tract then to the respiratory tract. Although the mechanisms underlying AM remain to be elucidated, microbiome alteration was considered as a critical cause. Skin and gut are the two main habitats of microbiota, and research in recent decades also indicated the presence of bacteria in lung. We here not only summarized the roles of skin, gut, lung microbiota in AD, FA, and AA, respectively, but investigated the crosstalk effects of microbiota in each anatomic site on remote organs, including microbiota-gut-skin axis, microbiota-gut-lung axis, and microbiota-skin-lung axis. In addition, we proposed the limitations of current research and the direction of future research in this field.