Lymphoid Organs Dr. Sami Zaqout
Cells of the Immune System Lymphocytes Plasma cells Mast cells Neutrophils Eosinophils Cells of the mononuclear phagocyte system
Distribution of cells of the immune system Lymphoid organs Lymphoid nodules Blood Lymph Epithelial tissues Connective tissues Lymph nodes Spleen Thymus Bone marrow
Main types of lymphocytes
Approximate Percentage of B and T Lymphocytes in Lymphoid Organs
Lymphoid Tissue Free cells Network of reticular fibrils Dense lymphoid tissue Loose lymphoid tissue Reticular cell The thymus is an exception
Lymphoid Tissue Lymphocytes Macrophage Reticular cell Trabecula
Lymphoid Tissue Lymphoid nodules Germinative center Primarily contain B lymphocytes
Mucosa-Associated Lymphoid Tissue (MALT) Located in the mucosa and submucosa of digestive, respiratory, and genitourinary tracts Collections of: Lymphocytes IgA-secreting plasma cells APCs Lymphoid nodules
Peyer's patches - M cells
Tonsils Palatine Pharyngeal Lingual
Palatine Tonsils
Pharyngeal Tonsil Situated in the superior posterior portion of the pharynx. It is covered by ciliated pseudostratified columnar epithelium. It is composed of pleats of mucosa and contains diffuse lymphoid tissue and lymphoid nodules. It has no crypts its capsule is thinner than the capsule of the palatine tonsils.
Lingual Tonsils Smaller and more numerous than the palatine and pharyngeal tonsils. They are situated at the base of the tongue They are covered by stratified squamous epithelium. Each lingual tonsil has a single crypt.
Thymus The thymus is a lymphoepithelial organ located in the mediastinum. It attains its peak development during youth. Has a dual embryonic origin: Lymphocytes (mesenchymal) Epithelial primordium (endoderm)
Thymus The thymus has a connective tissue capsule that penetrates the parenchyma and divides it into incomplete lobules Each lobule has Peripheral dark zone known as the cortex Central light zone called the medulla
Thymus Cortex The cortex is composed of: T cell precursors (thymocytes) Epithelial reticular cells Macrophages.
Thymus Cortex The epithelial reticular cells are stellate cells with lightstaining oval nuclei. They are usually joined to similar adjacent cells by desmosomes. Bundles of intermediate keratin filaments (tonofibrils) in their cytoplasm are evidence of the epithelial origin of these cells.
Thymus Medulla The medulla contains: Epithelial reticular cells Differentiated T lymphocytes Structures called thymic corpuscles or Hassall corpuscles.
Thymus Hassall corpuscles contain flattened epithelial reticular cells that are arranged concentrically and are filled with keratin filaments. They sometimes calcify.
Vascularization of the Thymus Arterioles and capillaries in the thymus are surrounded by processes of epithelial reticular cells. Thymus capillaries have a nonfenestrated endothelium and a very thick basal lamina creating the thymic blood barrier. The thymus has no afferent lymphatic vessels. The few lymphatic vessels encountered in the thymus are all efferent.
Role of the Thymus in T Cell Differentiation The thymus is the site of the terminal differentiation and selection of T lymphocytes. The thymus reaches its maximum development in relation to body weight immediately after birth. It undergoes involution after attaining its greatest size in puberty, but continues to produce lymphocytes until old age.
Lymph Nodes Elongated or kidney-shaped organs. Have a convex surface that is the entrance site of lymphatic vessels. Have a concave depression, the hilum, through which arteries and nerves enter and veins and lymphatic vessels leave the organ. A connective tissue capsule surrounds the lymph node, sending trabeculae into its interior.
Lymph Nodes
Lymph Node Outer Cortex 1. A diffuse population of cells composed mainly of T lymphocytes and reticular cells. 2. Lymphoid nodules, with or without germinative centers, formed mainly by B lymphocytes. 3. Areas of loose lymphoid tissue situated immediately beneath the capsule, called the subcapsular sinuses. 4. Intermediate or radial sinuses that run between lymphoid nodules.
Lymph Node Inner Cortex The inner cortex or paracortical region does not have precise boundaries with the outer cortex and contains few, if any, nodules but many T lymphocytes.
Lymph Node Medulla 1. Medullary cords: Are branched cordlike extensions of dense lymphoid tissue that arise in the inner cortex. They contain primarily B lymphocytes and often plasma cells and macrophages.
Lymph Node Medulla 2. Medullary sinuses: They contain lymph, lymphocytes, often many macrophages, and sometimes even granulocytes if the lymph node is draining an infected region. These sinuses join at the hilum delivering the lymph to the efferent lymph vessel of the lymph node.
Lymph Circulation
Role of Lymph Nodes in the Immune Response Because lymph nodes are distributed throughout the body, lymph formed in tissues must cross at least one node before entering the bloodstream. All antigens have the opportunity to be presented to B lymphocytes and to T helper and T cytotoxic lymphocytes, to initiate an immune response. The lymph node is an important site of lymphocyte proliferation (for instance, of B cells in the germinative centers) as well as of transformation of B lymphocytes into plasma cells.
Recirculation of Lymphocytes Lymphocytes that leave the lymph nodes by efferent lymphatic vessels eventually reach the bloodstream. They may then leave the blood vessels by entering the tissues and return to another lymph node by a lymph vessel. They may also return to a lymph node by crossing the walls of specific blood vessels, the high endothelial venules (HEVs), present in lymph nodes. High endothelial venules are also present in other lymphoid organs, such as the appendix, tonsils, and Peyer's patches, but not in the spleen.
Functions of Lymph Nodes Filtration of lymph. Formation of lymphocytes Immunological Functions of Lymph Nodes: A) Humoral Immunological Function B) Cell-mediated Immunological Function
Spleen The spleen is the largest accumulation of lymphoid tissue in the body. It is the only one interposed in the blood circulation. It is an important defense against antigens that reach the blood circulation. It is also the site of destruction of aged erythrocytes. It is a production site of activated lymphocytes, which are delivered to the blood.
General Structure
Splenic Pulp White Pulp Red Pulp Periarterial lymphatic sheath Lymphoid nodules Splenic cords Billroth's cords Sinusoids blood
Splenic Pulp
Splenic Pulp
Splenic Pulp
Sinusoids Elongated endothelial cells line the sinusoids of the spleen with the long axes parallel to the long axes of the sinusoids. These cells are enveloped in reticular fibers set primarily in a transverse direction.
Closed and Open Blood Circulation in the Spleen
Functions of the Spleen Phagocytosis and Immunological Defense Destruction of Erythrocytes
Differences Between Lymph Nodes And Spleen Lymph Nodes 1. Lymph nodes are multiple, present in groups all over the body 2. They filter the lymph. 3. They have many afferent and efferent lymph vessels. 4. Covered with fascia. 5. Capsule is thin, and not adherent. 6. Trabeculae are thin, short and arise from the capsule. 7. Lymphatic nodules are arranged into cortex and medulla. Spleen 1. The spleen is a single organ present in the abdomen. 2. It filters the blood. 3. It has few lymph vessels in the capsule and trabeculae. 4. Covered with peritoneum. 5. Capsule is partially thick, adherent and rich in smooth muscles 6. Trabeculae are thick, long and arise from the hilum and capsule. 7. Lymphatic tissues are the white and the red pulps ( not arranged in cortex and medulla).
Differences Between Lymph Nodes And Spleen Lymph Nodes 8. In the cortex there are the lymphatic nodules with apparent germinal centres but with no central arterioles. 9. Presence of cortical and medullary lymphatic sinuses. 10. Presence of medullary lymphatic cords. 11. Cells : are mainly lymphocytes, plasma cells and macrophages. 12. Functions: Humoral and cellmediaetd immunological functions. Spleen 8. The white pulps are scattered in the spleen. They contain central arterioles but their germinal centres are not apparent. 9. Presence of blood sinusoids all over the spleen. 10. Presence of Billroth cords or red pulps. 11. Cells : are mainly RBCs, Leucocytes, plasma cells and macrophages. 12. Function: Humoral and cellmediated immunological functions.