Lymphatic System Overview

Lymphatics Capillaries in Tissue

• The lymphatic system is responsible for maintaining fluid balance, absorbing lipids, and defending against pathogens.
• About 3 liters of fluid enter the lymphatic capillaries each day, which will not be reabsorbed into the blood, potentially leading to edema and tissue damage.
• This fluid is called lymph and is filtered by lymph nodes to remove pathogens.

Lymph Formation and Movement

• Fluid moves from blood capillaries into tissues, and from tissues into lymphatic capillaries, forming lymph.
• Overlapping epithelial cells of the lymphatic capillary allow fluid to enter easily but prevent its return to the tissues.
• Valves in the lymphatic vessels ensure unidirectional flow of lymph.

Parts of the Lymph Node

• Each lymph node is surrounded by a dense connective tissue capsule.
• Trabeculae extending from the capsule subdivide the lymph node into compartments containing lymphatic tissue and lymphatic sinuses.
• Lymphatic tissue consists of lymphocytes and other cells aggregated into lymphatic nodules.
• Lymphatic nodules containing rapidly dividing lymphocytes are called germinal centers.
• Lymphatic sinuses are spaces between the lymphatic tissue containing macrophages on a network of fibers.

Tonsils

• There are three groups of tonsils: palatine, pharyngeal, and lingual.
• Palatine tonsils, located on each side of the posterior opening of the oral cavity, are commonly called "the tonsils."
• Pharyngeal tonsils are located near the internal opening of the nasal cavity and are called adenoids when enlarged.
• Enlarged pharyngeal tonsils can interfere with breathing.
• Lingual tonsils are found on the posterior surface of the tongue.

Lymph Nodes

• Lymph nodes are rounded structures varying in size and located near lymphatic vessels.
• Lymph filters through lymph nodes before entering the blood.
• Superficial aggregations of lymph nodes are located in the groin (inguinal), axilla (armpit), and neck (cervical).

The Spleen

• The spleen filters blood instead of lymph.
• It has a capsule of dense connective tissue and a small amount of smooth muscle.
• Trabeculae from the capsule divide the spleen into compartments containing two types of lymphatic tissue: white pulp and red pulp.
• White pulp surrounds arteries and contains lymphocytes.
• Red pulp is associated with veins and is filled with macrophages, red blood cells, and enlarged capillaries connecting to the veins.

The Thymus

• The thymus is the site of T cell maturation.
• It is a bilobed gland located in the superior mediastinum, dividing the thoracic cavity into left and right parts.
• Each lobe of the thymus is surrounded by a connective tissue capsule, with trabeculae dividing it into lobules.
• Near the capsule and trabeculae, lymphocytes are numerous and form dark-staining areas called the cortex.
• The medulla, the lighter-staining central portion of the lobules, has fewer lymphocytes.

Overview of the Lymphatic System

• Lymphatic capillaries and vessels remove fluid from tissues.
• Lymph flows to lymphatic vessels with valves to prevent backflow.
• Lymph is filtered by lymph nodes and then flows back into the lymphatic vessels.
• Lymph from the right side of the body (upper extremity only) flows to the lymphatic duct; lymph from the left side flows to the thoracic duct.
• Both ducts drain into the subclavian vein, then into the superior vena cava (SVC), and finally into the heart.
• Specialized lymphatic vessels called lacteals in the small intestine absorb lipids, resulting in chyle due to its high lipid content.
• Chyle flows into the thoracic duct, then into the subclavian vein, SVC, and heart.
• The spleen filters blood and provides a site for lymphocyte response to infections.
• Pre-T cells originate in red bone marrow and migrate to the thymus for maturation into T cells.
• B cells develop and mature in red bone marrow, then circulate to other lymphatic tissues with T cells.

Immunity

• Immunity is the body's ability to resist damage from foreign substances.
• It provides protection against microbes, toxins, and cancer cells.

Types of Immunity

• Innate immunity is present at birth and defends against any pathogen.
• Adaptive immunity is acquired after birth and specifically recognizes antigens.

Innate Immunity

• Physical barriers like skin and mucous membranes act as the first line of defense.
• Chemical mediators include lysozyme, mucous membranes, histamine and interferons, all contributing to defense against pathogens.
• White blood cells (WBCs) are also part of innate immunity.

Adaptive Immunity

• Adaptive immunity is characterized by specific recognition and response to individual antigens.

Origin and Development of Lymphocytes

• Stem cells in red bone marrow give rise to all blood cells, including pre-T and pre-B cells.
• B cells mature in the red bone marrow and are responsible for antibody-mediated immunity.
• T cells mature in the thymus and are responsible for cell-mediated immunity and antibody-mediated immunity.

Antigen Recognition

• Lymphocytes have antigen receptors on their surface: B-cell receptors (BCRs) on B cells and T-cell receptors (TCRs) on T cells.
• Each receptor binds only with a specific antigen.
• When antigen receptors bind to their specific antigen, the lymphocyte is activated, initiating adaptive immunity.

The MHC Molecule

• Major histocompatibility complex (MHC) molecules contain binding sites for antigens, are specific for certain antigens, and present processed antigens on the cell membrane.
• MHC molecules interact with antigen receptors on B or T cells, stimulating an immune response.

Antibody-Mediated Immunity

• Antibody-mediated immunity is characterized by the production of antibodies in response to specific antigens.
• Antibodies are proteins released into the bloodstream, where they bind to antigens.
• This interaction neutralizes antigens or targets them for destruction by other immune cells.

Cytokines

• Cytokines are proteins released by cells that regulate the activity of neighboring cells.
• Interleukin 1, a cytokine released by macrophages, stimulates helper T cells.