The production, maturation, and function of cells of the immune system occur to a great extent in two overlapping organ systems, 1) the lymphoid system consisting of lymphocytes and their supporting structures and 2) the RES consisting of macrophages and related mononuclear phagocytes (Fig. 1-1). In postnatal life, bone marrow is the principal source of pluripotent stem cells that produce precursors of cells that operate in host defense (Fig. 1-2). The development of each type of leukocyte is precisely controlled and the controls account for the great specificity of the defense system and the fact that untoward immunologic reactions are relatively uncommon.

Figure 1-1

Major organs in the lymphoid and reticuloendothelial systems.

Figure 1-2

Production of blood cells from pluripotent stem cells in the bone marrow.