Chapter 10 - Immune System

Abstract

The immune system is composed of the bone marrow, thymus, spleen, lymph nodes, and various components of the mucosa-associated lymphoid tissue (MALT). The immune system organs are immature at birth and develop over the first 6 weeks of postnatal life and beyond. The primary lymphoid organs (bone marrow and thymus) are the first organs to complete postnatal development. The second phase of postnatal development occurs in the gut-associated lymphoid structures (mesenteric lymph nodes and Peyer’s patches). The third stage of postnatal development occurs in the somatic (peripheral) lymph nodes, spleen, and nasopharynx-associated or bronchus-associated lymphoid tissues. Postnatal development of secondary immune system organs continues beyond formation of the basic organs, and is determined to a substantial degree by the antigenic environment. The aim of this chapter is to describe the postnatal histological development of the immune system organs and highlight important developmental landmarks in each organ through postnatal development.