Chapter 11, Problem 1LO

Summary Introduction

To identify: The structures that constitute the lymphatic system and their general functions.

Concept introduction: A complex network of cells, tissues, and organs that works together to protect the body and fight against pathogens or foreign substances are collectively known as immune system. Lymphatic system consists of a network of vessels, tissue, and ducts that carries a clear fluid (lymph) to all body parts.

Explanation of Solution

The lymphatic system is an essential part of the immune system that constitutes the lymphatic vessels and lymphoid tissue, which includes pharyngeal tonsils, palatine, spleen, lymph nodes, bone marrow, and thymus gland.

Organs of lymphatic system:

Primary lymphoid organs

• Bone marrow:

  The spongy tissue called bone marrow is present in the center of bones (cancellous or spongy). This organ is an important site for the production of blood cells (hematopoiesis). It is the major site where B lymphocytes mature.

• Thymus:

  The thymus is the primary lymphoid organ. It is situated in the neck of vertebrates. Two identical lobes are found in the thymus. They are located in the anterior superior mediastinum, behind the sternum and in front of the heart. The thymus plays a fundamental role in the mechanism of immunity against infections. Before birth, they are the final site for the development of lymphocytes. It secretes hormones after birth, which facilitate the formation of mature T-lymphocytes. These T-lymphocytes help to attack the foreign cells and act as regulators for the immune system.

Secondary lymphoid organs

• Spleen:

  Spleen is the secondary lymphoid organ. It is an organ that is present in the upper left part of the abdomen. It helps to remove particulate matter and filter blood. It helps to remove antibody-coated bacteria and blood cells from the lymph node circulation by producing antibodies in its white pulp. This organ has many functions. They are as follows:

• Defense: The spleen is the site for phagocytic clearance of the bloodstream. The microbes are eliminated from the blood by macrophages and they are destroyed through phagocytosis process.
• Hematopoiesis: The spleen can act as a hematopoietic organ. The maturation and activation of lymphocytes and monocytes occur in the spleen. The erythrocytes are also produced in the spleen after birth.
• Blood reservoir: The spleen has a large amount of blood in venous sinuses and in pulp.
• Red blood cell and platelet destruction: These functions are mediated by macrophages. This organ destroys the platelets and old erythrocytes in the spleen through phagocytosis process.
• Lymph nodes:

  The lymph nodes are the secondary lymphoid organs present in the immune system. The lymph nodes are small glands that are situated throughout the body and are linked with the help of lymphatic vessels. These organs are the main sites of B and T lymphocytes. These organs act as a defense system. Before entering circulation, they filter the foreign particles from the lymph fluid. The lymph nodes are concentrated with lymphocytes (B-lymphocyte, T-lymphocyte, and other white blood cells) that could assess for the presence of foreign pathogenic materials like bacteria, virus, or fungi.

• Tonsils:

  Tonsils are soft tissues found in the lymphatic system. The pharyngeal and palatine tonsils contain lymphoid tissue which is present at the back of the throat. The tonsils can serve as sentinels that protect the lungs and digestive system from bacterial infection. They are the first line of defense during the attack of foreign organisms; thus, cause infections (tonsillitis).