Lymphoid Tissues

Learning objectives:

Know the structure and function of the lymphoid tissues and organs: (including afferent and efferent flow and specialized vasculature)

• tonsil
• diffuse lymphatic tissue and lymph nodules
• lymph nodes
• thymus
• spleen

Components of the Lymphatic System
Lymphoid tissue is not one of the primary or basic tissue types of the body but is a variety of connective tissue or, in the case of the thymus, epithelial tissue.  Lymphoid tissue is divided into central and peripheral types, which are either encapsulated or unencapsulated.

Central lymphoid tissue includes the thymus (encapsulated) and bone marrow (Bursal equivalent), which are the sites where development takes place.  In the thymus, T cells; and in the bone marrow, B cells (as well as monocytes, erythrocytes, granulocytes and megakaryocytes).

Peripheral lymphoid tissue includes:

1. diffuse lymphatic tissue: accumulations of unencapsulated lymphocytes in the lamina of various organs, which have a constant close relationship to an epithelium including the tonsils, gut, respiratory tract, etc.
2. lymph nodes (encapsulated):  the only lymphatic organs that are interposed in the course of lymphatic vessels.  They possess both afferent and efferent lymphatics.
3. lymphatic tissue of the spleen (encapsulated), which is interposed in the blood circulation.

Lymphatic vessels form an alternative channel by means of which the following are removed from the connective tissue spaces: fluid, particulate matter, protein that escapes from blood capillaries, ingested fats absorbed across the intestinal surface, disease microorganisms, other antigenic material which may penetrate the skin and other protective surfaces, and cells both normal and cancerous. This proteinaceous fluid, variably rich in particles and cells, is termed lymph and is filtered and cleansed in lymph nodes before being returned via the thoracic duct and the right lymphatic duct to the circulating blood in the great veins.

Lab Activities

1. Tonsils
2. Gut Associated Lymohatic Tissue (GALT)
3. Lymph Nodes
4. Thymus (or Thymus Gland)
5. Spleen
6. Electron Micrographs
7. Questions
8. Answers