| Lymphatic tissues or organs are involved in the defence of the organism against bacteria, viruses and immunogenic antigens. The present chapter is limited to three well-encapsulated lymphatic organs: the thymus, lymph nodes and spleen. In other sites lymphatic tissue may have a more diffuse distribution as in the bone marrow cavities, or may have a vaguely nodular appearance as in the wall of the gastro-intestinal tract (e.g., tonsils and Peyer’s patches in the small intestine) and along the wall of the appendix. Diffuse lymphatic tissue is also seen along the respiratory passages.
The thymus is of epithelial origin. It produces thymic lymphocytes, or T-lymphocytes. Well vascularized with blood vessels, it is free of internal lymphatic vessels. Numerous lymph nodes are present along the broad network of lymphatic vessels. They are structured to interact with antigens that enter the lymphatic circulation. They have separate blood and internal lymphatic circulations. In contrast to lymph nodes, the spleen has an elaborate blood circulation but no internal lymphatic vessels. This organ is structured to filter the blood. Bone marrow is an important lymphatic tissue, which will be described in Chapter 7. Bone marrow contributes to the production of B-lymphocytes but also produces red and white blood cells and blood platelets. |
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