Sanne Hoeks

General Introduction 19 1 changes in next generations. Genes that are sensitive for epigenetic changes are for example genes coding for enzymes involved in detoxing effects of oxidative stress (like cigarette smoke and pollution) and genes coding for proteins involved in metabolic and immunological processes. Already before birth both maternal and paternal environmental factors like cigarette smoke, nutrients, medication and maternal disease influence the development of the fetal immune system via epigenetic changes. Some of these maternal factors are known to induce stable epigenetic changes in gene expression and to pass these adaptations on to their offspring in next generations. 54, 55 Therefore, the epigenome can be conceived of as a series of switches that cause various parts of the genome to be expressed or not. 56 The fetal and neonatal period may be particularly important for setting these switches and meddling factors like infectious pathogens, nutrients, chemicals but also the presence of commensals might induce long lasting structural or functional effects in this crucial developmental period. These negative influences may set switches wrongly and lead to morbidities later in childhood or adulthood.