The Neonatal Adaptive Immune System

After completing this article, readers should be able to: 1. Describe the basic sequence of events that initiates an immune response. 2. Understand the basic differences between neonatal and adult T-cell function. 3. Describe the accumulation of maternal immunoglobulin G in the fetus. 4. Understand the barriers to strong B-cell function in the neonate. In 1960, the famous immunologist Peter Medawar was awarded a Nobel Prize for experiments describing the relative ease with which fetal immune systems develop tolerance to foreign antigens. (1) Neonates, therefore, were viewed as “immunodeficient,” and the neonatal period was believed to be a unique window in time during which any antigen encountered by the immune system would be tolerated and included in the repertoire of “self” antigens. However, more recent experiments have proven that, under the right conditions, the neonatal immune system generates responses similar in strength and quality to those of adults, leading some to speculate that when it comes to the immune system, there is “nothing special about the neonate.” (2) The truth lies somewhere in the middle of these two extreme views. The success of modern vaccination programs confirms that infants can mount significant immune responses. However, in the face of other immunologic challenges, such as group B streptococcal infections, infants demonstrate obvious weaknesses compared with adults. What do we know about the neonatal immune system? In what ways is it different from the adult immune system? What are the cellular and molecular mechanisms for these differences? This review summarizes the development of T- and B-cell immunity in the human neonate, with emphasis on the clinical implications of data. The adaptive immune system consists of T cells, B cells, and their products. (3) T cells are lymphocytes defined by their surface expression of the T-cell receptor (TCR) and are divided into subsets based on their surface …