The spleen is an organ shaped like a shoe that lies relative to the 9th and 11th ribs and is located in the left hypochondrium and partly in the epigastrium. Thus, the spleen is situated between the fundus of the stomach and the diaphragm. The spleen is very vascular and reddish purple in color; its size and weight vary. A healthy spleen is not palpable.
The spleen develops in the cephalic part of dorsal mesogastrium (from its left layer; during the sixth week of intrauterine life) into a number of nodules that fuse and form a lobulated spleen. Notching of the superior border of the adult spleen is evidence of its multiple origin (see the image below). 
The spleen's 2 ends are the anterior and posterior end. The anterior end of the spleen is expanded and is more like a border; it is directed forward and downward to reach the midaxillary line. The posterior end is rounded and is directed upward and backward; it rests on the upper pole of the left kidney.
The spleen's 3 borders are the superior, inferior, and intermediate. The superior border of the spleen is notched by the anterior end. The inferior border is rounded. The intermediate border directs toward the right.
The 2 surfaces of the spleen are the diaphragmatic and visceral. The diaphragmatic surface is smooth and convex, and the visceral surface is irregular and concave and has impressions. The gastric impression is for the fundus of the stomach, which is the largest and most concave impression on the spleen. The renal impression is for the left kidney and lies between the inferior and intermediate borders. The colic impression is for the splenic flexure of the colon; its lower part is related to the phrenicocolic ligament. The pancreatic impression for the tail of the pancreas lies between the hilum and colic impression (see the image below).
The hilum can be found on the inferomedial part of the gastric impression (see the image above). The hilum transmits the splenic vessels and nerves and provides attachment to the gastrosplenic and splenorenal (lienorenal) ligaments.
The spleen is surrounded by peritoneum and is suspended by multiple ligaments, as follows:
The gastrosplenic ligament extends from the hilum of the spleen to the greater curvature of the stomach; it contains short gastric vessels and associated lymphatics and sympathetic nerves.
The splenorenal ligament extends from the hilum of the spleen to the anterior surface of the left kidney; it contains the tail of the pancreas and splenic vessels.
The phrenicocolic ligament is a horizontal fold of peritoneum that extends from the splenic flexure of the colon to the diaphragm along the midaxillary line; it forms the upper end of the left paracolic gutter.
The visceral surface of the spleen contacts the following organs:
Anterior surface of the left kidney
Splenic flexure of the colon
The fundus of the stomach
Tail of the pancreas
The diaphragmatic surface is related to the diaphragm; the diaphragm separates the spleen from the pleura and the lung.
The splenic artery supplies blood to the spleen. This artery is the largest branch of the celiac trunk and reaches the spleen's hilum by passing through the splenorenal ligament. It divides into multiple branches at the hilum. It divides into straight vessels called penicillin, ellipsoids, and arterial capillaries in the spleen.
The splenic circulation is adapted for the separation and storage of the red blood cells. The spleen has superior and inferior vascular segments based on the blood supply. The 2 segments are separated by an avascular plane.
Its terminal branches aside, the splenic artery also gives off branches to the pancreas, 5-7 short gastric branches, and the left gastro-omental (gastroepiploic) artery (see the image below).
The spleen is marked on the left side of the back with the long axis of the 10th rib. The upper border is marked along the upper border of the ninth rib; the lower border, along the 11th rib. The medial end lies 5 cm from the midline. The lateral extension ends at the midaxillary line. 
The splenic vein provides the principal venous drainage of the spleen. It runs behind the pancreas (after forming at the hilum) before joining the superior mesenteric vein behind the neck of the pancreas to form the portal vein. The short gastric, left gastro-omental, pancreatic, and inferior mesenteric veins are its tributaries.
Proper splenic tissue has no lymphatics; however, some arise from the capsule and trabeculae and drain to the pancreaticosplenic lymph nodes.
The spleen is made up of the following 4 components:
Supporting tissue is fibroelastic and forms the capsule, coarse trabeculae, and a fine reticulum.
The white pulp consists of lymphatic nodules, which are arranged around an eccentric arteriole called the Malpighian corpuscle.
The red pulp is formed by a collection of cells in the interstices of the reticulum, in between the sinusoids. The cell population includes all types of lymphocytes, blood cells, and fixed and free macrophages. The lymphocytes are freely transformed into plasma cells, which can produce large amounts of antibodies and immunoglobulins (see the image below).
The vascular system traverses the spleen and permeates it. 
Natural and Pathophysiologic Variants
See the list below:
Accessory spleens or splenunculi are natural anatomic variants formed from nodules that fail to fuse during development. These are found in various locations such as the gastrosplenic ligament, splenorenal ligament, gastrophrenic ligament, and gastrocolic ligament. They have also been reported to have been found in the broad ligament of the uterus and in the spermatic cord. 
Pathophysiologic anatomic variants include splenomegaly, asplenia, and autosplenectomy. Splenomegaly is the enlargement of the spleen. It occurs due to various conditions, such as infections (eg, malaria, kala azar), malignancies (eg, lymphomas, leukemias), and other conditions (eg, portal hypertension). The spleen then projects toward the right iliac fossa in the direction of axis of the 10th rib.
Asplenia is a rare condition in which a congenital absence of the spleen occurs.
Autosplenectomy is a condition in which splenic infarction occurs due to sickle cell anemia.
Functions of the Spleen
After antigenic stimulation, increased formation of plasma cells for humoral responses and increased lymphopoiesis for cellular responses occurs.
One of the spleen's most important functions is phagocytosis. The spleen is a component of the reticuloendothelial system. The splenic phagocytes include reticular cells, free macrophages of the red pulp, and modified reticular cells of the ellipsoids. Phagocytes in the spleen remove debris, old and effete red blood cells (RBCs), other blood cells, and microorganisms, thereby filtering the blood. Phagocytosis of circulating antigens initiates the humoral and cellular immune responses.
The spleen is an important hematopoietic organ during fetal life; lymphopoiesis continues throughout life. The manufactured lymphocytes take part in immune responses of the body. In the adult spleen, hematopoiesis can restart in certain diseases such as chronic myeloid leukemia and myelosclerosis.
Storage of red blood cells
The RBCs are stored in the spleen. Approximately 8% of the circulating RBCs are present within the spleen; however, this function is seen better in animals than humans.