 is a part of the immune system and  plays an important role in defending the body against harmful substances, such as bacteria, viruses, foreign bodies, and other micro-organisms that can cause diseases and abnormal conditions. Definition: What it is: A widely distributed system consisting of all the cells able to ingest bacteria or colloidal particles etc.

The reticulo-endothelial system also helps the body to rid itself of old or abnormal cells, such as old RBCs and plays a role in creating new RBCs. Other functions: The reticuloendothelial system consists of cells that are phagocytic. Phagocytic cells have the ability to surround and destroy foreign substances and old cells. Phagocytic cells are produced by the bone marrow and are found throughout the body in the bloodstream and in tissues. Composition:

 The lymphatic system is a part of the circulatory system,  comprising a network of conduits (a pipe or channel that carries liquid to or from a place) called lymphatic vessels  that carry a clear fluid called lymph (from Latin lympha "water goddess")  directionally towards the heart. Lymphatic organs play an important part in the immune system. The system also includes the spleen, thymus, bone marrow and the lymphoid tissue associated with the digestive system. Definition:

 It is responsible for the removal of interstitial fluid from tissues  It absorbs and transports fatty acids and fats as chyle from the digestive system  It transports white blood cells to and from the lymph nodes into the bones  The lymph transports antigen-presenting cells (APCs), such as dendritic cells, to the lymph nodes where an immune response is stimulated. Functions:

 It is concerned with immune functions in defending the body against the infections and spread of tumors.  It consists of connective tissue with various types of WBCs involved in it. But they also contain other types of cells for support. It is found in many organs, particularly in the lymph nodes & in the lymphoid follicles associated with the digestive system such as the tonsils. Locations:

 The lymphoid tissue may be primary, secondary, or tertiary depending upon the stage of lymphocyte development and maturation. (The tertiary lymphoid tissue typically contains far fewer lymphocytes, and assumes an immune role only when challenged with antigens that result in inflammation. It achieves this by importing the lymphocytes from blood and lymph.) Types:

Primary lymphoid organs:  The central or primary lymphoid organs generate lymphocytes from immature progenitor cells (can divide only a limited number of times).  The thymus and the bone marrow constitute the primary lymphoid tissues involved in the production and early selection of lymphocytes.

It is exemplified by the lymph nodes, and the lymphoid follicles in tonsils, Peyer's patches, spleen, adenoids, skin, etc. that are associated with the mucosa-associated lymphoid tissue (MALT) Secondary lymphoid organs:  Secondary or peripheral lymphoid organs maintain mature naive (still virgin) lymphocytes and initiate an adaptive immune response.  The peripheral lymphoid organs are the sites of lymphocyte activation by antigen  Secondary lymphoid tissue provides the environment for the foreign or altered native molecules (antigens) to interact with the lymphocytes.

 A lymph node is an organized collection of lymphoid tissue, through which the lymph passes on its way to returning to the blood.  Lymph nodes are located at intervals along the lymphatic system.  Several afferent lymph vessels bring in lymph, which percolates (like the way how water passes through sand) through the substance of the lymph node, and is drained out by an efferent lymph vessel.

 Substance of a lymph node consists of lymphoid follicles in the outer portion called the "cortex," which contains the lymphoid follicles, and  an inner portion called "medulla," which is surrounded by the cortex on all sides  except for a portion known as the "hilum." The hilum presents as a depression on the surface of the lymph node, which makes the otherwise spherical or ovoid lymph node bean-shaped. The efferent lymph vessel directly emerges from the lymph node here. The arteries and veins supplying the lymph node with blood enter and exit through the hilum. Structure of lymph node:

o are a dense collection of lymphocytes, o the number, size and configuration of which change in accordance with the functional state of the lymph node. For example, the follicles expand significantly upon encountering a foreign antigen. What are lymph follicles? dense collection of Lymphocytes Lymphoid Follicles Substance of the lymph node Lymph Node

Location of Lymph nodes: Lymph nodes are particularly numerous in-  the mediastinum (part of the thoracic cavity containing the heart except the lungs) in the chest,  neck,  pelvis,  axilla (armpit),  inguinal (groin) region and  in association with the blood vessels of the intestines.

 The thymus gland is a specialized organ that forms part of the immune system.  In the thymus gland, lymphoid cells undergo a process of maturation and education prior to release into the circulation. This process allows T cells to develop the important attribute known as self tolerance. It continues to function throughout the life.

 The thymus gland is found in the thorax in the anterior mediastinum.  It gradually enlarges during childhood but after puberty it undergoes a process of involution resulting in a reduction in the functioning mass of the gland.  The following cell types are present: 1.lymphoid cells 2.epithelial cells 3.macrophages 4.other supporting cells

 to transform lymphocytes (white blood cells developed in the bone marrow) into T-cells. These cells are then transported to various lymph glands, where they play an important part in fighting infections and disease. Swelling of lymph glands and fever are a signal that immune cells are multiplying to fight off invaders of the body: bacteria, fungi, viruses or parasites.

 All the cells of the immune system are initially derived from the bone marrow (this process is known as “Hematopoiesis”)  During foetal development hematopoiesis occurs initially in yolk sac and para-aortic mesenchyme and later in the liver and spleen.  This function is taken over gradually by the bone marrow. During hematopoiesis, bone marrow-derived stem cells differentiate into either mature cells or into precursors of cells that migrate out of the bone marrow to continue their maturation in thymus. How Hematopoiesis occurs?

 produces B cells, natural killer cells, granulocytes and immature thymocytes in addition to red blood cells and platelets. That is why it functions both as a primary and secondary lymphoid organ.  also contains antibody secreting plasma cells, which have migrated from the peripheral lymphoid tissue. Function of Bone Marrow

 Structurally similar to lymph node it filters circulating blood to remove worn out RBCs and pathogens.  Is soft purple gray in color located in the left upper quadrant of the abdomen.  It is a highly vascular lymphoid organ  It plays an important roles in RBSs integrity and has immune function.  It holds a reserve of blood in case of hemorrhagic shock.  It is one of the centers of activity of the RES Despite its importance, there are no tests specific to splenic function

White pulp: Thick sleeves (part) of lymphoid tissue, that provides the immune function of the spleen. Red pulp: surrounds white pulp, composed of venous sinuses filled with whole blood and Splenic cords of reticular connective tissue rich in macrophages. Structural Functions of Spleen

 As the organ is directly connected to blood circulation, it responds faster than other lymph nodes to blood-borne antigens  Reservoir of lymphocytes in white pulp  Destruction and processing of antigens  Site for Phagocytosis of bacteria and worn-out blood cells (Slow blood flow in the red pulp cords allows foreign particles to be phagocytosed) Immune Functions of Spleen:

 Site of B cell maturation into plasma cells, which synthesize antibodies in its white pulp and initiates humoral response  Removes antibody-coated bacteria  It contains (in its blood reserve) half of the body monocytes within the red pulp, upon moving to injured tissue (such as the heart), turn into dendritic cells and macrophages that promoting tissue healing. Immune Functions of Spleen:……. contd

Location: The tonsils (palatine tonsils) are a pair of dense compact bodies of lymphoid tissue located at the rear of the throat (pharynx).

 Each tonsil is composed of tissue similar to lymph nodes, covered by pink mucosa.  The tonsils are part of the lymphatic system, which helps to fight infections. However, removal of the tonsils does not seem to increase susceptibility to infection.  Tonsils vary widely in size and swell in response to infection.

Both tonsils and adenoid are part of the Waldeyer ring, which is a ring of lymphoid tissue found in the pharynx. The lymphoid tissue in this ring provides defense against pathogens. The Waldeyer ring is involved in the production of immunoglobulins and the development of both B cells and T cells. Adenoid:  The adenoid is a median mass of MALT.  It is situated in the roof and posterior wall of the nasopharynx.

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