Cells and Organs of the Immune System
The mammalian immune system consists of several organs, tissues and cell types circulating in the blood and lymph. The cellular elements of blood (Table 2–I) are red cells (erythrocytes), white cells (leukocytes) and platelets (thrombocytes). All formed elements of blood derive from a common precursor: the pluripotent stem cell (Figure 2–1). The differentiation of stem cells and maturation of their derivatives is called hemopoiesis. Blood Cells.
Phagocytosis, inflammation, allergy
Cell-mediated immunity and regulation of immune responses<italic>Hemopoiesis</italic>. In the upper left of the figure is the pluripotent stem cell, the single cell capable of giving rise to all formed elements of blood. The reflexive arrow indicates that this cell may also generate new pluripotent stem cells. This cell first commits to either lymphoid or erythroid-myeloid differentiation (<italic>CFU-GEMM, CFU=colony-forming unit</italic>). The myeloid precursor cell gives rise to further committed stem cell stages of differentiation for each non-lymphoid blood cell lineage. These intermediate stem cells proceed through several differentiative steps (not shown) generating mature blood cells (at the periphery of the figure). The “decision” of a stem cell to begin differentiating along a particular pathway appears to be a random event. However, progress along a pathway is regulated by an extremely complex interplay of bone marrow tissue elements and soluble factors (see text). Shown in the figure are some of the factors driving hemopoiesis and the differentiative pathways they regulate. (<italic>BFU-E=burst-forming unit-erythroid</italic>, see <xref ref-type="table" rid="tab2_II">Table 2–II</xref> for other abbreviations. Adapted from <xref ref-type="bibr" rid="ref2_10">Furman and Crist, 1992</xref>.)