The lymphatic system is a subsystem of the circulatory system.

Its three principal functions are:

	to collect and return interstitial fluid, including plasma protein to the blood, and thus help maintain fluid balance,
	to defend the body against disease by producing lymphocytes,
	to absorb lipids from the intestine and transport them to the blood.

In comparison to the cardiovascular system the lymphatic system has not in the past been the focus of much research. However it’s important role in the body’s immune system has meant that it has increasingly become the focus of research in more recent times. It is only developed in the higher vertebrates which have an extensive vascular system. Such a system has to have a high intravascular pressure if the blood is to be adequately transported. This results in fluid seepage from the capillaries and it is thought that the lymphatic system originally evolved to combat this problem. Lymphatic tissue is a type of connective tissue characterised by large numbers of lymphocytes. The stroma (framework) of lymphatic tissue is a network of reticular fibres. Connective tissue cells including fihroblasts and macrophages are present.

Lymphatic tissue is organised in 3 different ways.

	Diffuse lymphatic tissue
	Lymph nodules
	Lymph nodes

Small amounts of diffuse lymphatic tissue are found in virtually every organ of the body.

In addition the spleen and the thymus gland are considered to be lymphatic tissue.