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 meshworlc of reticular cells and reticular fibers, and a rich supply of macrophages. These contents allow the spleen to monitor the blood immunologically, much as the macrophages of the lymph nodes monitor the lymph.

Most of the spleen consists of splenic pulp. Splenic pulp, in turn, is divided into two functionally and morphologically different regions: white pulp and red pulp, based on the color of fresh sections. White pulp appears as circular or elongated whitish gray areas surrounded by red pulp.

The spleen is enclosed by a dense connective tissue capsule from which trabeculae extend into the parenchyma of the organ (Fig. 13.29). The connective tissue of the capsule and trabeculae contains myofibroblasts. These contractile cells also produce extracellular connective tissue fibers. In

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negative selection positive selection type I ere capsule type VI ere Hassall's corpuscle postcapillary venule

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