Spleen: Definition, Structure and Functions

The spleen is a large, encapsulated, bean-shaped organ that is situated on the left side of the body below the diaphragm. The spleen contains T and B lymphocytes as well as many phagocytes and is a major component of the mononuclear phagocyte system. Although the structure resembles that of the large lymph nodes, the spleen differs from a lymph node in having no lymphatic drainage, and also in containing large numbers of red cells.

Structure of Spleen

• It is a dark purple-coloured organ, which lies in the left hypochondriae region of the abdomen, between the fundus of the stomach and the diaphragm.
• It varies in size and weight during the lifetime of an individual but in an adult is usually about 12 cm long, 8cm broad and 3-4 cm thick weighing about 200gm.
• The spleen has diaphragmatic and visceral surfaces. The diaphragmatic surface is in contact with the inner surface of the diaphragm.
• The spleen has an outer coat of peritoneum which is firmly adherent to the internal fibro-elastic coat or splenic capsule that dips into the organ, forming trabeculae.

• The spleen has a spongy interior called splenic pulp. The splenic pulps are of two kinds:

1. White Pulp:

• It consists of periarteriolar sheaths of lymphatic tissue with enlargements called splenic lymphatic follicles containing rounded masses of lymphocytes.
• These follicles are center of lymphocytes production called primary lymphoid follicles, composed mainly of follicular dendritic cells (FDC) and B cells.
• They are visible to the naked eye in freshly cut surface of the spleen as whitish dots against the dark red background of red pulp.
• The white pulp forms ‘islands’ within a meshwork of reticular fibers containing red blood cells, macrophages and plasma cells (red pulp).

2. Red Pulp:

• It consists of numerous sinusoids containing blood, separated by a network of perivascular tissue which is referred to as the splenic cords.
• The splenic cords contain numerous microphages abd are the site of intense phagocytes activity.
• They also contain numerous lymphocytes, which are derived from the white pulp.

Functions of Spleen

1. The main immunological function of the spleen is to filter the blood by trapping bloodborne microbes and producing an immune response to them. It is particularly important for B cell responses to polysaccharide antigens.
2. The spleen is formed partly by lymphatic tissue which produces T lymphocytes and B lymphocytes.
3. Due to the presence of lymphoid reticulo-endothelial tissue, the spleen is involved in producing antibodies and antitoxin.
4. In the foetus, the spleen acts as an important haemopoitic organ.
5. It also removes damaged red blood cells and immune complexes.
6. It can act as an erythropoietic organ which acts as a reservoir of erythrocytes or a reservoir for blood.
7. Those individuals who have had their spleens removed (splenectomized) have a greater susceptibility to infection with encapsulated bacteria, and are at increased risk of severe malarial infections, which indicates its major importance in immunity.

References

1. Lydyard, P.M., Whelan,A.,& Fanger,M.W. (2005).Immunology (2 ed.).London: BIOS Scientific Publishers.
2. Playfair, J., & Chain, B. (2001). Immunology at a Glance. London: Blackwell Publishing.
3. Tuitui, R., & Suwal, D. S. (2010). Human Anatomy and Physiology. Kathmandu: Vidyarthi Prakashan.