Almost all vertebrate animals have an internal body organ called the spleen. It is located in the left upper quadrant of the abdomen, and to the left of the stomach in humans.
Anatomically, the spleen may vary in size and shape. But in healthy adult people it is shaped like a fist, purple, and more or less 11 centimeters (4.3 inches) long. In general, its weight is about 150 grams (5.3 oz.) and is usually found below the 9th to the 12th thoracic ribs in humans, thus it is protected by the rib cage and cannot be easily palpated unless it is enlarged (a condition called splenomegaly).
The spleen is part of the lymphatic system. And like the thymus, it only has efferent lymphatic vessels, meaning that it only has blood vessels carrying blood away from it.
The spleen has two basic parts: the red pulp and the white pulp. These two parts are the sites where the spleen’s two most important functions take place.
The red pulp is composed of sinuses or sinusoids which contain blood. This section of the spleen is responsible for the filtration of red blood cells; at the same time it also acts as a reserve of monocytes, a type of white blood cells whose primary functions include responding to infection by migrating to sites of inflammation or injury.
The white pulp is the area which contains nodules known as Malpighian corpuscles, which are composed of lymphoid follicles, containing high amount of B lymphocytes; and the periarteriolar lymphoid sheaths (PALS) which are rich in T lymphocytes. The white pulp is responsible for the body’s active immune response which could either be humoral (mediated by antibodies produced in the B lymphocytes) or cell-mediated (involving the T lymphocytes).
Although of less significance particularly in healthy adults, the spleen also plays a role in other processes, such as follows:
• Production of antibodies like opsonins, properdin, and tuftsin.
• Production of red blood cells (erythropoiesis). Although the main site for blood cell production in adults is the bone marrow, the spleen plays vital erythropoietic functions up to the fifth month of gestation. But this function stops right after birth, except in some blood disorders. The spleen remains a hematopoietic organ (produces blood cells) in the sense that it retains its ability to produce lymphocytes.
• Storage of red blood cells as well as other formed elements. In humans, the spleen does not only act as a pool of red and white blood cells, but it can also store platelets during an emergency.
• Storage of half the monocyte reserves of the body so that these cells can immediately migrate to the site of injury or infection where they transform into dendritic cells and macrophages which fight the invading microorganisms and help in wound healing.
The spleen indeed plays several key functions in the body that surgical removal of this organ can cause a modest increase in white blood cells and platelets in the circulation; reduced responsiveness to the action of some vaccines; and heightened vulnerability to infections caused by bacteria and protozoa in particular.