In the Western world about 1 in 500 adults have a primary immunodeficiency (PID). While some affected individuals do not have any severe clinical symptoms during their life, others will have severe complications almost immediately after birth. Over 170 different forms of PID have been described, with involvement of more than 120 distinct genes. During the last decade many genetic defects have been discovered within these PID’s, not only providing explanations for clinical symptoms but also giving more insight in immune development
With the discovery of every new genetic defect comes a unique opportunity to study the immune system. Having the chance to study a genetic knockout, a unique experiment of nature, helps us understand the complexity of the human immune system. But before being able to study what the effect of this knockout will be on for instance lymphocyte development, maturation, and function, it is essential to have a good understanding of what occurs in healthy children.