The microRNA network, and specifically the microRNA miR-29a, is critical for protecting the thymus against inappropriate atrophy during infections, according to a report published in Nature Immunology. The thymus, the dedicated organ for T cell development, undergoes gradual and progressive atrophy with age, as well as periodical and reversible atrophy driven by pathogen infections. This process is known as thymic involution. Age-driven involution is mediated by the sensitivity of the thymic epithelial cells to sex hormones, while infection-induced involution is controlled by the sensitivity of these cells to interferon-alpha (IFN-alpha), a molecular mediator of the immune response. Adrian Liston and colleagues show that microRNAs, which are small, non-coding RNAs that modulate protein production, reduce the sensitivity of the thymic epithelium to IFN-alpha signals and are critical for protecting the thymus against infection-induced involution.