FIGURE 14.28 • Schematic drawing of the major steps in thymic education. The process of multipotential lymphatic stem cell (CFU-L) molecule, indicating the middle stage of T-cell differentiation. As maturation progresses, the cells express TCRs, CD3, CD4, and CD8 molecules. These cells are then presented with epithelioreticular (erc) cells. If the lymphocyte recognizes self MHC and self or foreign antigen, then it will survive the selection (positive TCR selection); if not, death of the cell will occur. Cells that pass the positive selection test leave the cortex and enter the medulla. Here they undergo to self-antigen displayed by self MHC are typeV eliminated (negative selection). Cells that survive erc that selection then become either cytotoxic CD8 T lymphocytes or helper CD4 T lymphocytes.
CD8 These cells are now ready for the immune response; they leave the thymus from the medulla and enter the blood circulation. Hormonal substances secreted by type VI epithelioreticular cells within the thymic (Hassall’s) corpuscle promote the process of thymic cell education. Note the distribution of all six types of epithelioreticular cells.
cytotoxic CD8 T lymphocytes (by losing CD4 and retaining CD8) or helper CD4 T lymphocytes (by losing CD8 and retaining CD4). This stage is called the single-positive stage of T-cells differentiation. Now the cells leave the thymus by passing from the medulla into the blood circulation. The process of thymic cell education is promoted by substances secreted by the epithelioreticular cells, including interleukins (IL-4 and IL-7), colony-stimulating factors, and interferon .
The spleen is about the size of a clenched fist and is the largest lymphatic organ. It is located in the upper left quadrant of the abdominal cavity and has a rich blood supply.
The spleen filters blood and reacts immunologically to blood-borne antigens.
The spleen has both morphologic and immunologic filtering functions. In addition to large numbers of lymphocytes, it contains specialized vascular spaces or channels, a meshwork of reticular cells and reticular fibers, and a rich supply of macrophages and dendritic cells. These contents allow the spleen to monitor the blood immunologically, much as the macrophages and dendritic cells of the lymph nodes monitor the lymph.
The spleen is enclosed by a dense connective tissue capsule from which trabeculae extend into the parenchyma of the organ (Fig. 14.29). The connective tissue of the capsule