Lymphoid System and Immunity Lecture Notes

Components and Functions of the Lymphoid System

• Definition of Components:    

•  * Lymph: This is the fluid that is collected from the interstitial spaces of tissue and subsequently returned to the bloodstream.    

•  * Lymphatic Vessels: These structures are responsible for the transport of lymph throughout the system.   

•   * Lymphoid Tissue: This tissue is characterized by the presence of clusters of lymphocytes.     

• * Lymphoid Organs: These consist of concentrated collections of cells that are set off from surrounding organs by connective tissue (CT).

• Primary Functions:    

•  * Fluid Recovery: The system reabsorbs excess fluid from the tissues and returns it to the blood. A failure in this function leads to a condition known as lymphedema.  

•    * Immune Surveillance: The system acts as a monitoring network, reporting any impurities or biological invasions to the body's defenses.    

•  * Lipid Absorption: Specialized structures called lacteals absorb dietary lipids that are not able to be absorbed by standard intestinal blood capillaries.

Characteristics and Formation of Lymph

• Physical Properties: Lymph is typically clear and colorless. Notably, it contains no protein.

• Biological Composition: It contains large lymphocytes, macrophages, hormones, bacteria, viruses, cellular debris, and potentially traveling cancer cells.

• Chyle: This is a specific type of milky fluid found in the small intestine that is incorporated into the lymphatic system.

• Lymphatic Capillaries: These vessels penetrate almost all tissues except for cartilage, bone, bone marrow, and the cornea.     * They are structured as sacs of thin endothelial cells that overlap one another.     * They are closed at one end.    

•  * Lymphatic Anchoring Filaments: These structures are essential for preventing the capillary sacs from collapsing.

Lymphatic Vessel Structure and the Route of Flow

• Valve Mechanisms: The overlapping edges of the endothelial cells in the capillaries act as valvular flaps that open and close based on pressure gradients.    

•  * When tissue pressure is high, the flaps open inward, allowing fluid to enter.    

•  * When pressure within the lymphatic capillary is high, the flaps close (outward), which prevents the fluid from leaking back into the tissue spaces.

• Anatomy of Large Lymphatic Vessels:     * Tunica Interna: The innermost layer consisting of endothelium and valves.     * Tunica Media: The middle layer composed of elastic fibers and smooth muscle.     * Tunica Externa: The thin outermost connective tissue layer.

• The Route to the Bloodstream: The progression follows a specific hierarchy:     * Lymphatic capillaries $\rightarrow$ collecting vessels $\rightarrow$ lymphatic trunks $\rightarrow$ collecting ducts $\rightarrow$ subclavian veins.

• Collecting Ducts Detail:     
  * **Right Lymphatic Duct:** Converges from the right jugular, subclavian, and bronchomediastinal trunks, draining the right upper limb, thorax, head, brain, and spinal cord into the right subclavian vein.
  * **Thoracic Duct:** The larger duct begins below the diaphragm and collects chyle from lumbar and intestinal trunks, ascending to drain lymph from the left side into the left subclavian vein.

• Mechanisms of Lymph Flow: Because the system lacks a central heart pump, it operates at a lower pressure and speed than venous blood. Flow is maintained by:     * Contractions of the lymphatic vessels.     * The presence of valves within the lymphatic vessels.     * Skeletal muscle contractions (the skeletal muscle pump).

The Glymphatic System of the Brain

• Definition: The Glymphatic system refers to the glial-associated lymphatic system specific to the brain.

• Process of Filtration and Waste Removal:     * Phase 1: Cerebrospinal fluid (CSF) filters through perivascular feet into the nervous tissue.     * Phase 2: This fluid mixes with tissue fluid, effectively rinsing the brain of metabolic waste.     * Phase 3: The waste is transported via the perivascular space to the subarachnoid space.     * Phase 4: Waste exits through a "dual outflow system." It can either enter the bloodstream via arachnoid granulations or enter the lymphoid system via lymphatic vessels in the subarachnoid space.     * Phase 5: The fluid is eventually carried to the deep lymph nodes located in the neck.

Lymphoid Cells and the Macrophage System

• T Lymphocytes: These cells mature in the thymus.     * Helper T Cells: Responsible for activating other immune cells to provide a response to pathogens.     * Cytotoxic T Cells: These cells directly destroy infected or malignant body cells.

• B Lymphocytes: These cells mature in the bone marrow.     * Upon activation, they differentiate into plasma cells.     * Plasma Cells: These cells are responsible for secreting antibodies.

• The Macrophage System:     * Functions to break down tissue debris, including old/expired red blood cells (RBCs) and dead or dying neutrophils.     * They secrete growth factors necessary for healing and regenerating tissue.     * They function as antigen-representing cells.

Lymphoid Tissues and Organs

• Diffuse Lymphoid Tissue: This is a simple arrangement of scattered lymphocytes, dendritic cells, and other immunity cells. It is found in mucous membranes such as the respiratory and digestive tracts. This is collectively referred to as MALT (Mucosa-Associated Lymphoid Tissue), which fights local infections.

• Lymphoid Nodules: These are clusters that can grow, shrink, or appear/disappear as pathogens invade. They are found in lymph nodes, tonsils, and the appendix. Aggregated lymphoid nodules are specifically located in the ileum (the distal portion of the small intestine).

• Categorization of Organs:     

• * Primary Lymphoid Organs: Red bone marrow and the thymus. These are the sites where T and B cells mature and become immunocompetent.     

• * Secondary Lymphoid Organs: Lymph nodes, tonsils, and the spleen. Immunocompetent cells migrate to these locations.

• Red Bone Marrow Details: Contains sinusoids. The structure consists of endothelial cells and reticular cells forming a supportive reticular connective tissue. Reticular cells secrete colony-stimulating factors that trigger leukocyte formation. As marrow ages, it transforms into yellow marrow.

• Thymus Details: Organized into a cortex and a medulla. It features a network of interconnected thymic epithelial cells. The medulla is populated with T cells.

• Lymph Nodes Details:     

• * Function: To clean lymph and serve as a site for T cell and B cell activation.  

•    * Structure: Features an indentation called the hilum; it is encased in a fibrous capsule with trabeculae that divide the interior into compartments. The subcapsular sinus contains reticular fibers, macrophages, and dendritic cells.    

•  * Parenchyma: Includes a C-Shaped Cortex and a Medulla. The cortex contains germinal centers where B cells multiply and differentiate. The medulla consists of a network called medullary cords containing lymphocytes, plasma cells, macrophages, and reticular cells/fibers.     * Flow: Lymph enters via afferent lymphatic vessels $\rightarrow$ subcapsular sinus $\rightarrow$ cortex + medulla $\rightarrow$ exits through $3$ efferent lymphatic vessels. Lymph is cleansed as it flows from one node to the next.

• Tonsils: Lymphoid tissue located in the pharynx for protection against ingested or inhaled pathogens. Covered by epithelium with deep pits called tonsillar crypts. Crypts contain food debris, dead leukocytes, bacteria, and antigenic chemicals.

• Spleen: The largest lymphoid organ, located in the left hypochondriac region. It has indentions for the gastric and renal areas. It consists of Red Pulp (sinusoids with erythrocytes) and White Pulp (lymphocytes and macrophages).

The Immune System and Modes of Defense

• General Definition: A system of numerous cells and mechanisms defending the body against pathogens.

• Three Modes of Defense:     1. Physical Barriers: The skin and mucous membranes.     2. Nonspecific Actions: Defenses against pathogens that bypass the first physical barrier.     3. Adaptive Immunity: A system that remembers specific invaders to defeat them more effectively in the future.

• Innate vs. Adaptive Immunity:     * Innate Immunity: Present from birth. It is fast-acting, causes fever and inflammation, but lacks a memory of specific pathogens.     * Adaptive Immunity: This system "remembers" and responds specifically. It utilizes B and T cells and provides separate immunity for each unique pathogen.

• Lymphocyte Development:     * Lymphocytes start as Hematopoietic Stem Cells (HSC), which give rise to lymphocyte-colony forming units that produce B and T cells.     * Cell survival depends on becoming immunocompetent and surviving negative selection.     * B cells undergo immunocompetence and negative selection within the red bone marrow, subsequently staying there or moving to mucous membranes.

• Lymphocyte Activation: B cells operate "at a distance." They seek help from T cells and then transform into plasma cells secretes antibodies.