The spleen is the largest lymphoid organ in the body. It is the only lymphoid organ that filters the blood, allowing for B and T cells to be activated in response to antigens in the blood. The spleen also contains abundant macrophages that function to remove old red blood cells from the circulation and destroy them.

• Examine this section of spleen and the image at the right, and identify the capsule, connective tissue trabeculae, and the overall organization of the spleen into white pulp and red pulp.

• The arterial supply to the white and red pulp is shown in the diagram at the right. The components are difficult to observe in routine histological preparations, but you should understand the blood flow and try to identify the major structures on your slide. Click the image for the expanded view.

• On the slide and image below, identify central arterioles associated with the white pulp lymphoid follicles, the T cell-rich periarteriolar lymphoid sheathes (PALS), as well as the marginal zone of these follicles. Try to find a vascular sinus showing the unusual nature of the lining endothelial cells or stave cells.
• In the red pulp, identify the splenic cords (of Billroth), seen in the lower right image, and the venous sinuses. The latter are small in this section of spleen, but can be detected by their contents of red blood cells. Sheathed vessels and sinuses are seen more readily in this section of spleen, which is stained with a special stain that also demonstrates elastic fibers in the trabeculae.

Clinical note: Because its capsule is thin and the blood-filled red pulp is very delicate, the spleen is easily ruptured by injury to the abdomen. This is a very serious event that can lead to death from the loss of blood into the abdominal cavity. Pictured to the right is a fatal splenic rupture.
 

Now try this try this short quiz to practice what you've learned.

Next is the digestive system.