General Structure of spleen
The spleen is the largest accumulation of lymphoid tissue in the body. The spleen is surrounded by a capsule of dense connective tissue from which emerge trabeculae, which divide the parenchyma, or splenic pulp, into incomplete compartments. Large trabeculae originate at the hilum, on the medial surface of the spleen; these trabeculae carry nerves, arteries and veins.

In humans, the connective tissue of the capsule and trabeculae contains only a few smooth muscle cells, contrary to what occurs in several animals (eg, horses, dogs, and cats). 

Splenic Pulp
The spleen is composed of a network of reticular tissue that contains reticular cells, many lymphocytes and other blood cells, macrophages, and APCs. The splenic pulp has two components, the white pulp and the red pulp. These names derive from the fact that on the surface of a cut through an unfixed spleen, white spots (lymphoid nodules) are observed within a dark red tissue that is rich in blood. The white pulp consists of the periarterial lymphatic sheath (PALS) and the lymphoid nodules, whereas the red pulp consists of splenic cords (Billroth's cords) and blood sinusoids.

White Pulp
The splenic artery divides as it penetrates the hilum, branching into trabecular arteries of various sizes that follow the course of the connective tissue trabeculae. When they leave the trabeculae to enter the parenchyma, the arteries are immediately enveloped by a sheath of T lymphocytes, the periarterial lymphatic sheath (PALS), which is part of the white pulp. These vessels are known as central arteries or white pulp arteries. After coursing through the parenchyma for variable stretches, the PALS receive large collections of lymphocytes ––mostly B cells––forming lymphoid nodules. In these nodules the artery, which has now turned into an arteriole, occupies an eccentric position but is still called the central artery. During its passage through the white pulp, the artery also divides into numerous radial branches that supply the surrounding lymphoid tissue.

Surrounding the lymphoid nodules is a marginal zone consisting of many blood sinuses and loose lymphoid tissue. A few lymphocytes but many active macrophages can be found there. The marginal zone contains an abundance of blood antigens and thus plays a major role in the immunological activities of the spleen.

After leaving the white pulp, the sheath of lymphocytes slowly thins and the central artery (arteriole) subdivides to form straight penicillar arterioles with an outside diameter of approximately 24 μm. Near their termination, some of the penicillar arterioles are surrounded by a thick sheath of reticular cells, lymphoid cells, and macrophages. How the blood is delivered to the trabecular veins is not exactly known and will be discussed later. 
The figure-2 above shows lymphoid nodule of the spleen surrounded by red pulp. A germinative center (1) and the (eccentric) central artery (2), which is characteristic of the spleen, are clearly visible.

Red Pulp
The red pulp is composed of splenic cords and sinusoids. The splenic cords contain a network of reticular cells supported by reticular fibers. The splenic cords contain T and B lymphocytes, macrophages, plasma cells, and many blood cells (erythrocytes, platelets, and granulocytes).

The splenic cords are separated by irregularly shaped wide sinusoids. Elongated endothelial cells line the sinusoids of the spleen with the long axes parallel to the long axes of the sinusoids. These cells are enveloped in reticular fibers set primarily in a transverse direction, much like the hoops on a barrel. 

Surrounding the sinusoid is an incomplete basal lamina. Because the spaces between the endothelial cells of the splenic sinusoids are 2–3 μm in diameter or smaller, only flexible cells are able to pass easily from the red pulp cords to the lumen of the sinusoids. Unfortunately, because the lumen of sinusoids in the red pulp may be very narrow and the splenic cords are infiltrated with red blood cells, microscopic observation of a spleen section is not always easy; observation of PALS may also be difficult.

Closed and Open Blood Circulation in the Spleen
The manner in which blood flows from the arterial capillaries of the red pulp to the interior of the sinusoids has not yet been completely explained. Some investigators suggest that the capillaries open directly into the sinusoids, forming a closed circulation in which the blood always remains inside the vessels. Others maintain that the prolongations of the penicillar arteries open into the splenic cords, and the blood passes through the space between the cells to reach the sinusoids (open circulation)

From the sinusoids, blood proceeds to the red pulp veins that join together and enter the trabeculae, forming the trabecular veins. The splenic vein originates from these vessels and emerges from the hilum of the spleen. The trabecular veins do not have individual muscle walls. They can be considered channels hollowed out in the trabecular connective tissue and lined by endothelium.

Functions of the Spleen
Phagocytosis and Immunological Defense
Because of its strategic position in the blood circulation, the spleen is able to filter, phagocytize, and mount immunological responses against blood-borne antigens. The spleen contains all the components (B and T lymphocytes, APCs, and phagocytic cells) necessary for this function. 

The white pulp of the spleen is an important production site of lymphocytes, which then migrate to the red pulp and reach the lumen of the sinusoids, where they enter the blood circulation. Inert particles are also intensely phagocytized by spleen macrophages.

However, it is not a vital organ (not necessary to sustain life).

Also Read:
Gross Anatomy of Spleen 

Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):
Junqueira's Basic Histology: Text and Atlas, 13th Edition
Wheater's Functional Histology: A Text and Color Atlas, 5th Edition
Wheater's Functional Histology Basic Histology
Wheater's Functional Histology Basic Histology
Wheater's Functional Histology Basic Histology