The spleen. A. Section of spleen The spleen. A. Section of spleen. Areas of red and white pulp are intermixed beneath the dense capsule. Central arteries (CA) are associated with the lymphocytes in white pulp, and a germinal center (GC) is seen at the center of a secondary follicle, indicating an advanced B-cell response. Thick trabeculae penetrate the organ and carry vascular elements. B. Central artery. The central artery is surrounded by a periarterial lymphatic sheath (PALS), which consists predominantly of T cells. The lower cell density of red pulp is evident. C. Red and white pulp. The central artery (CA) is eccentric, in this case due to the proliferation of B cells and the associated germinal center (GC), indicating active production of plasma cells and antibody secretion. By the open theory of circulation, blood from some of the central arteries is released at the marginal zone area and must travel through the red pulp to large splenic sinuses, where it re-enters the circulation. D. Blood flow in the spleen. T cells maintain a symmetric relationship in PALS around the central arteries, whereas areas of rapid B-cell expansions in follicles grow away from the artery. Penicillar arterioles branch from the central artery and either connect directly to splenic sinuses (closed circulation) or terminate at the marginal zone, dumping blood into red pulp tissue (open circulation). Red blood cells are processed by macrophages as they traverse red pulp to remove inclusions and destroy old cells. Source: IMMUNE SYSTEM, The Big Picture: Histology Citation: Ash R, Morton DA, Scott SA. The Big Picture: Histology; 2017 Available at: https://accessphysiotherapy.mhmedical.com/DownloadImage.aspx?image=/data/books/2058/ashhist_ch9_f007d.png&sec=155784482&BookID=2058&ChapterSecID=155784404&imagename= Accessed: November 14, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved