The early years of life are crucial for the maturation and balance of our immune system. Many factors can influence this, including how and when we are exposed to different microorganisms. Changing exposure to different microbes is thought to be one reason for the recent marked increase in various inflammatory diseases. For example, allergies in children have increased significantly and are now a global health problem. Our research has previously shown a clear association between the presence of lactobacilli in the very early gut flora, and early infection with two very common herpes viruses and reduced risk of allergy development. We have also been able to demonstrate that these types of early microbial stimulation affect the immune system in a way that may partly explain the 'allergy-protective' effect. Here we want to go further and study in detail the interactions between these microorganisms and the immune system at different ages and investigate how it relates to allergy development. The project will provide us with information on basic immunological processes during childhood and how different types of microorganisms can affect children's immunological profile and allergy development. The studies are experimental in nature but have a strong clinical basis. In our studies we use cell lines, animal models and experimental systems as well as material from a very well characterized allergy cohort, where we can correlate all findings to clinical parameters and allergy status up to 10 years of age.