All cells that participate in immunity originate initially from the multipotential or pluripotential stem cell in the bone marrow. The pluripotential stem cell then differentiates into a Myeloid Progenitor Cell or a Lymphoid Progenitor cell. Each progenitor cell then further differentiates into the various blood cell lines, and then each matures. The mature cells then migrate to the peripheral blood or various organs in the body, depending on their function.
Unlike red blood cells and platelets, which function in the bloodstream, all cells which participate in immunity, including the white blood cells seen in the peripheral blood, function in the various tissues. The image at the right is a simplified diagram of hematopoiesis.
1. Häggström, Mikael, Rad, A. "Simplified Hematopoiesis." Wikimedia Commons, 21 Jul 2009, https://commons.wikimedia.org/wiki/File:Hematopoiesis_simple.svg
