Spleen

The spleen is a ductless, vertebrate gland that is not necessary for life but is closely associated with the circulatory system, where it functions in the destruction of old red blood cells and removal of other debris from the bloodstream, and also in holding a reservoir of blood. It is regarded as one of the centres of activity of the reticuloendothelial system. Until recently, the purpose of the spleen was not known. It is increasingly recognised that its absence predisposes to certain infections.

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Gland: A gland is an organ in an animal's body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland)....

Circulatory system: The circulatory system or cardiovascular system is the organ system which circulates blood around the body of most animals....

Red blood cell: Red blood cells are the most common type of blood cell and are the vertebrate body's principal means of delivering oxygen from the lungs or gills to body tissues via the blood....