SPLEEN

The spleen is an ingeniously designed filter for the blood and a site of immune responses to blood-borne antigens. Normally in the adult it weighs about 150 gm and is enclosed within a thin, glistening, slate-gray connective tissue capsule. Its cut surface reveals extensive red pulp dotted with gray specks, which are the white pulp follicles. These consist of an artery with an eccentric collar of T lymphocytes, the so-called periarteriolar lymphatic sheath. At intervals this sheath expands to form lymphoid nodules composed mainly of B lymphocytes, which are capable of developing into germinal centers identical to those seen in lymph nodes in response to antigenic stimulation


The spleen has four functions that impact disease states:

	1.	Phagocytosis of blood cells and particulate matter. As will be discussed under the hemolytic anemias.. red cells undergo extreme deformation during passage from the cords into the sinusoids. In conditions in which red cell elasticity is decreased, red cells become trapped in the cords and are more readily phagocytosed by macrophages. Splenic macrophages are also responsible for “pitting” of red cells, the process by which inclusions such as Heinz bodies and Howell-Jolly bodies are excised, and for the removal of particles, such as bacteria, from the blood.
	2.	Antibody production. Dendritic cells in the periarterial lymphatic sheath trap antigens and present them to T lymphocytes. T- and B-cell interaction at the edges of white pulp follicles leads to the generation of antibody-secreting plasma cells, which are found mainly within the sinuses of the red pulp. The spleen seems to be an important source of antibodies directed against platelets and red cells in immune thrombocytopenia purpura and immunohemolytic anemias, both discussed in Chapter 14 .
	3.	Hematopoiesis. Splenic hematopoiesis normally ceases before birth, but can be reactivated in severe anemia. As we have seen, the spleen is also a prominent site of extramedullary hematopoiesis in myeloproliferative disorders, such as chronic myeloid leukemia.
	4.	Sequestration of formed blood elements. The normal spleen contains only about 30 to 40 mL of red cells, but this volume increases greatly with splenomegaly. The normal spleen also harbors approximately 30% to 40% of the total platelet mass in the body. With splenomegaly up to 80% to 90% of the total platelet mass can be sequestered in the interstices of the red pulp, producing thrombocytopenia. Similarly, the enlarged spleen can trap white cells and thereby induce leukopenia.