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One milliliter of human blood typically contains about 5 billion red blood cells and 7 million of the larger white blood cells (also called leukocytes). All of these cells originate from multipotent stem cells (constantly dividing undifferentiated cells that can form several different cell types; see Key Concept 19.1) in the bone marrow. There are two major families of white blood cells: lymphocytes and phagocytes (Figure 41.3). Lymphocytes include the B cells and T cells; they are smaller than other white blood cells and are not phagocytic. Phagocytes include most of the other cells shown in Figure 41.3, and as their name suggests, they are phagocytic. Each kind of white blood cell has specialized functions. Some phagocytes are also referred to collectively as granulocytes because they contain numerous granules (vesicles containing defensive molecules). Defensive proteins and signals play fundamental roles in the interactions and functioning of these cells.