Neonatal Immunity

The nature of neonatal immunity has defied simple definition, as evidenced by the ongoing evolution of our understanding of responses in early life. In the 1980s, neonates were considered to be immunodeficient, incapable of mounting significant responses of any type. Subsequently, in the 1990s and early 2000s, robust Th2 responses were identified in newborns, and the thinking shifted such that neonates were considered to be immunodeviant. Now, well into the twenty-first century, it is abundantly apparent that neonates are actually competent to mount a wide range of responses, from severely compromised to deviated to mature or supramature. While our understanding of how this arises is still underdeveloped, recent evidence indicates that a wealth of factors likely influence this variability in the responsiveness of neonates. These include cell types and immunomodulators that either exist exclusively in or are differently represented in early life. Many of these factors are negative regulators of immunity and may dominate in those instances of deficient or deviated responses. However, some of these factors can be thought of as positive regulators of immunity and may play important roles in generating protective responses in newborns. Thus, the outcome of challenge to the immune system in early life may be a reflection of the balance between these positive and negative influences.