Primary immunodeficiencies (PID) are a group of congenital disorders that are caused by different defects in immune system development and function. The symptoms of these diseases can be mild and the diagnosis belated, but in more serious cases an infant can have some serious or prolonged infections with opportunistic or atypical pathogens, that can lead to complications and even to the infant’s death. Besides infections, there are other signs which can lead to the suspicion of PID-s, for
example, the failure to thrive, serious skin rashes, hepatosplenomegaly, family anamnesis of PID, cytopenias, thymus aplasia or hypoplasia, granulomatous or autoimmune processes. Severe PID-s require an early diagnosis, as the results of one possible curative treatment with HSCT (hematopoietic stem cell transplantation) is shown to be best if patient with severe combined immune deficiency has been treated with HSCT before the age of 3,5 months. In Estonia, seven children have needed HSCT and one child needed thymus transplant during last nine years. In all patients HSCT has provided a significant improvement in health and quality of life.