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.
Connective tissue cells including fihroblasts and macrophages are present. Lymphatic tissue is organised in 3 different ways. 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.