There is an orderly development of the immune system during the intrauterine period. Pluripotent stem cells appear first in the yolk sac, then in the fetal liver, and finally the bone marrow (Fig. 1-2). Neutrophils, monocytes, and macrophages are produced during fetal life, but the mononuclear phagocytes do not mature until after birth. An epithelial thymus appears during the first few fetal weeks and then becomes populated with lymphocytes. Mature T and B cells appear in the blood soon thereafter, but they are largely not activated. Furthermore, IgG antibodies are usually not produced until after birth, and IgG antibodies to polysaccharide antigens do not appear until ~ 2 years of age. In addition, there are developmental delays in the production of certain cytokines, including GM-CSF, IL-10, TNF-α and IFN-γ.
Neonates have as many B and T cells in the peripheral blood as do adults, these cells in the peripheral lymphoid organs are not as well developed because of the paucity of prenatal antigenic stimuli. As antigen stimulation occurs, the T and B cell zones of the peripheral lymphoid organs are progressively populated and the products of these stimulated cells, such as antibodies, begin to appear. The sequence of immunoglobulin production is as follows: IgM production occurs first and is then followed by IgG and IgA. Systemic IgG antibodies to polysaccharides are not produced, however, until the child is 2 to 2.5 years old. The secretory component is produced at birth, but the main immunoglobulin in external secretions in the first few weeks of postnatal life is IgM. Subsequently, secretory IgA becomes the dominant immunoglobulin at mucosal sites