The spleen is interposed directly into the blood stream. Unlike the lymph nodes, it possesses neither afferent lymphatic vessels nor a lymphatic sinus system. One of the major functions of the spleen is to remove old and worn out red blood cells.
Spleen, Human (H&E) [#40]: The spleen is characterized by a robust capsule from which heavy irregular trabeculae of connective tissue and smooth muscle extend into the organ (arrow). From the appearance of fresh unfixed tissue, the spleen is divided into red and white pulp. The white pulp consists of periaterial lymphoid sheaths (PALS) which may also contain lymphoid follicles scattered irregularly throughout the organ. Characteristically, what is called the central artery can usually be located at the periphery of the follicle. Most of the spleen is red pulp which consists of splenic sinusoids and splenic cords which consist of various blood cells (mostly R.B.C.ís) in reticular stroma. Between the white pulp and red pulp is the marginal zone which consists of many sinuses and loose lymphoid tissue. Except at the hilum, the spleen is covered completely with peritoneum.