The hematopoietic system consists of the cascade of cells produced by the bone marrow and their specialized conducting and supporting systems. These systems include vascular endothelium, the connective tissue cells of the marrow, lymph nodes, and spleen, and organ-specific specialized cell populations. The hematopoietic cells consist of all differentiated products of pluripotent stem cells, including monocyte–macrophage and granulocytic cells, as well as RBC, platelet, and lymphocyte precursors. Unlike secondary lymphoid tissues (e.g., lymph nodes) that respond to antigenic stimulation, the thymus does not form lymphoid germinal centers. However, nonpathologic germinal centers can develop within the thymic vascular epithelial sheaths, as this unique location is immunologically “outside” the thymus. Body surfaces that interface with the “outside world,” including the skin, respiratory tract, and enteric system, have resident lymphoid populations and antigen-presenting cells vital in immune defense.