e do not grow absolutely, chronologically. We grow sometimes in one dimension, and not in another; unevenly. We grow partially. We are relative. We are mature in one realm, childish in another. The past, present and future mingle and pull us backward, forward, or fix us in the present. We are made up of layers, cells, constellations."-Anaïs Nin It has long been recognized that the developing immune system exhibits certain peculiarities when compared to the adult immune system. Nonetheless, many still regard the fetal immune system as simply being an immature version of the adult immune system. Here we discuss historical evidence as well as recent findings, which suggest that the human immune system may develop in distinct layers with specific functions at different stages of development.
A Brief Historical Perspective on the Development of Lymphocytes in MammalsOver two decades ago, a model was proposed by Leonore and Leonard Herzenberg which suggested that the immune system in mammals did not arise in a linear fashion from immaturity to maturity, as is often believed, but rather in distinct layers which could perform unique functions at different stages of development.1 This model was supported by a series of observations, made first in avian species 2,3 and later in the mouse, 4-10 that revealed the existence of unique waves of lymphocyte production during fetal and neonatal development.