In modern times, high-income countries are witness to steep increases in the prevalence of inflammatory disease, with urban populations additionally showing more allergies, autoimmune conditions, inflammatory bowel disease, and psychiatric conditions, including depression, anxiety, autism, and schizophrenia. Such illnesses have been linked to toxic exposures in urban environments, which offer novel ‘biomes’ of synthetic, microbial, and industrial substances that do not occur naturally. However, merely eliminating toxic exposures is insufficient for developing immune balance and tolerance.
Spending time in green spaces or aquatic areas appears to partially remedy the maladaptive immune stimulation received in man-made environments. Living close to natural areas is associated with reductions in mortality, cardiovascular illness, and psychiatric disturbances. These beneficial influences are likely mediated by psychological and immune-related mechanisms that can interact in complex ways.
This review analyzes extensive epidemiological and experimental data, and suggests that normal immune tolerance must be acquired through training, much as immunity is trained by infections. Dr. Rook argues that, historically, organisms normally encountered through social and environmental contacts served as immune educators, inducing tolerance and immunity through skin, respiratory, gut, and other routes of exposure. He states that “at birth the immune system is like a computer that contains programs (genetics) but almost no data,” and relies on vaginal birth and increasing contact with microbial diversity for successful training of immune balance. Examples of such immune educators include helminths, certain other parasites, dust, varied human and environmental microbiota, and common exposures to potential pathogens that confer immunity, like hepatitis A or Helicobacter pylori.