Introduction

The spleen is a lymphatic organ that plays a fundamental role in protecting the body from invading pathogens. Being an organ that is interposed in the blood stream, it also stands as the body's largest blood filter that furthermore brings contribution to detecting senescent, mechanically damaged and aberrant cells. The spleen combines the innate and adaptive immune system in a unique way, releasing an immediate innate reaction to microbial penetration, but also an adaptive immune response that involves the interaction of cells that recognize a particular antigen, implicating MHC molecules presented by antigen-presenting cells. Clinical manifestations of some hematologic conditions can be controlled by splenectomy. The use of this procedure, although, has been restricted due to many observations of overwhelming post-splenectomy bacterial infections in splenectomized patients. After splenectomy, the mechanisms that play a fundamental role in bacterial clearance are altered, leading to gram-positive, but also gram-negative sepsis. Subtotal splenectomy is, therefore, a logical alternative that controls the manifestations of hematologic diseases while maintaining splenic function.[1]

Structure of spleen and its cross section

Source:

Kuby, 5^th edition, Unit 2: Cells and organs of immune system, pg 50, Immunology.

In detail…

It is a large, ovoid secondary lymphoid organ situated high in the left abdominal cavity. Unlike the lymph nodes, the spleen is not supplied by lymphatic vessels. Instead, bloodborne antigens and lymphocytes are carried into the spleen through the splenic artery. Experiments with radioactively labeled lymphocytes show that more recirculating lymphocytes pass daily through the spleen than through all the

lymph nodes combined. The spleen is surrounded by a capsule that extends a number of projections (trabeculae) into the interior to form a compartmentalized structure. The compartments are of two types, the red pulp and white pulp, which are separated by a diffuse marginal zone. The splenic red pulp consists of a network of sinusoids populated by macrophages and numerous red blood cells (erythrocytes) and few lymphocytes; it is the site where old and defective red blood cells are destroyed and removed. Many of the macrophages within the red pulp contain engulfed red blood cells or iron pigments from degraded hemoglobin. The splenic white pulp surrounds the branches of the splenic artery, forming a periarteriolar lymphoid sheath (PALS) populated mainly by T lymphocytes. Primary lymphoid follicles are attached to the PALS. These follicles are rich in B cells and some of them contain germinal centers. The marginal zone, located peripheral to the PALS, is populated by lymphocytes and macrophages.

Blood-borne antigens and lymphocytes enter the spleen through the splenic artery, which empties into the marginal zone. In the marginal zone, antigen is trapped by interdigitating dendritic cells, which carry it to the PALS. Lymphocytes in the blood also enter sinuses in the marginal zone and migrate to the PALS. The initial activation of B and T cells takes place in the Tcell- rich PALS. Here interdigitating dendritic cells capture antigen and present it combined with class II MHC molecules to TH cells. Once activated, these TH cells can then activate B cells. The activated B cells, together with some TH cells, then migrate to primary follicles in the marginal zone. Upon antigenic challenge, these primary follicles develop into characteristic secondary follicles containing germinal centers (like those in the lymph nodes), where rapidly dividing B cells (centroblasts) and plasma cells are surrounded by dense clusters of concentrically arranged lymphocytes.

The effects of splenectomy on the immune response depends on the age at which the spleen is removed. In children, splenectomy often leads to an increased incidence of bacterial sepsis caused primarily by Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae. Splenectomy in adults has less adverse effects, although it leads to some increase in blood-borne bacterial infections (bacteremia).[2]

Reference:

[1] Tiron A, Vasilescu C, 2008 May-Jun;103(3):255-63. Role of the spleen in Immunity,Immunologic consequences of splenectomy, Abstarct, PubMed.

[2] Kuby, 5^th edition, Unit 2: Cells and organs of immune system, pg 49-50, Immunology.