Figure 1331

Schematic diagram of open and closed splenic circulation. In the open circulation, which occurs in humans, penicillar arterioles empty directly Into the reticular meshwork of the cords rather than connecting to the endothelium-lined splenic sinuses. Blood entering the red pulp then percolates through the cords and is exposed to the macrophages residing there. In the closed circulation, which occurs in other species, the penicillar arterioles empty directly into the sheathed capillaries splenic sinuses of the red pulp.

Macrophages recognize senescent or abnormal blood cells by several different mechanisms:

• Nonspecific mechanisms involve morphologic and biochemical changes that occur in aged erythrocytes; they become more rigid and are thus more easily trapped in the mesh of the red pulp.

• Specific mechanisms include opsonization of the cell membrane with anti-band 3 IgG antibodies, which trigger Fc receptor-dependent phagocytosis of erythrocytes.

In addition, specific changes in glycosylation of gly-cophorins (see page 217) in aging erythrocytes act as a recognition signal that triggers the elimination of senescent erythrocytes by macrophages.

Despite these important functions, the spleen is not essential for human life. It can be removed surgically, which is often done following trauma that causes intractable bleeding from the spleen. The removal and destruction of aging red blood cells then occurs in the bone marrow and liver.

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