Lymphatic System Overview

Lymphatic System Overview

• The lymphatic system consists of lymph, lymphatic vessels, and structures/organs containing lymphatic tissue. Lymph is a fluid. Lymphatic vessels transport lymph.
• The lymphatic system maintains fluid balance, transports dietary lipids, and carries out immune responses.

Lymphatic System - Components

• Lymph is a clear, yellowish fluid that forms when fluid from blood vessels seeps into tissues. It's primarily interstitial fluid with varying amounts of lymphocytes, bacteria, cellular debris, plasma proteins, and other cells. The body produces 3-4 liters of lymph daily.
• Lymphatic vessels include: lymphatic capillaries, lymphatic trunks, and lymphatic ducts.
• Primary lymphatic organs are sites where stem cells produce immunocompetent cells (i.e., bone marrow and thymus).
• Secondary lymphatic organs are sites where immune responses occur (i.e., lymph nodes, spleen, tonsils, and lymphatic nodules).

Lymphatic System - Primary Organs

• Red Bone Marrow: Produces immunocompetent B cells and pre-T cells, and mature lymphocytes (B-lymphocytes).
• Thymus: The pre-T cells migrate to the thymus where they become immunocompetent T cells. The thymus is a bilobed organ in the mediastinum. Each lobe consists of an outer cortex and a central medulla.

Lymphatic System - Thymus: Cortex

• The cortex is primarily composed of large numbers of T cells.
• Epithelial cells provide a framework for as many as 50 T cells and help educate pre-T cells through a process called positive selection.
• Dendritic cells and macrophages assist the maturation process by clearing debris. The processed T cells then migrate to the medulla.

Lymphatic System - Thymus: Medulla

• The medulla contains widely scattered, more mature T cells, along with dendritic cells, macrophages, and epithelial cells.
• It includes thymic corpuscles, also known as Hassall's corpuscles, which are believed to be aged and degenerated cells.

Lymphatic System - Bone Marrow

• Bone marrow is the site of hematopoiesis, where lymphocytes are produced.
• B cells develop fully within the bone marrow. Immature T cells (thymocytes) leave the bone marrow to mature in the thymus.

Lymphatic System - Secondary Organs

• Lymph Nodes: These are sometimes called the "filters of the lymph". They remove debris and pathogens from lymph. Lymph nodes range in size from 1-25mm.
• The lymph node structure includes a capsule, trabeculae (connective tissue extensions), reticular fibers & fibroblasts forming the stroma (supporting framework), and the parenchyma (functioning part). The parenchyma is divided into a cortex and medulla. The cortex has a paracortex region, predominantly containing T cells and dendritic cells. The medulla contains B cells and plasma cells.
• Lymph generally flows through the node in only one direction. It enters through afferent vessels, flows through sinuses, and exits through efferent vessels at the hilum.

Lymphatic System - Spleen

• The spleen is the largest single mass of lymphatic tissue. It measures approximately 12 cm in length and is located in the left hypochondriac region. It's sometimes called the "filter of the blood”.
• The spleen's parenchyma consists of white pulp (containing lymphatic tissue mostly lymphocytes and macrophages) and red pulp (containing blood filled venous sinuses and splenic cords composed of red blood cells, lymphocytes, macrophages, plasma cells & granulocytes).
• Within the spleen, blood flowing through the splenic artery enters the white pulp. B and T cells perform immune functions, macrophages remove pathogens by phagocytosis within the white pulp. The red pulp carries out functions related to blood cells such as: removal of defective/worn-out blood cells/platelets, blood cell storage, and hematopoiesis (during fetal development).

Lymphatic System - Lymphatic Nodules

• Lymphatic nodules are egg-shaped masses of lymphatic tissue not enclosed in a capsule. They are found throughout mucous membranes and include tonsils (pharyngeal, palatine, and lingual) and aggregated lymphatic follicles (Peyer's patches).

Lymphatic System - Lymph

• Lymph flows through numerous lymphatic vessels/capillaries which drain into lymphatic ducts. Lymph eventually returns to the bloodstream via the junction of the internal jugular and subclavian veins.

White Blood Cells (Leukocytes)

• These are a vital component of the blood, playing a crucial role in combating infections. They are categorized as either granular or agranular, based on their cellular structure.

• Granular Leukocytes: Includes neutrophils, eosinophils, and basophils.

• Neutrophils: Are phagocytes with trilobed nuclei, being the most prevalent leukocyte type. They rapidly respond to tissues and release enzymes (like lysozyme) to destroy bacteria and pathogens.

• Eosinophils: Primarily combat parasitic infections and allergic reactions, releasing enzymes to break down chemical mediators.

• Basophils: Crucial for short-term inflammatory responses, particularly to allergies, releasing histamine causing vasodilation.

• Agranular Leukocytes: Include lymphocytes and monocytes.

• Monocytes: Are phagocytes that differentiate into macrophages. They clean up cellular debris and microbes via phagocytosis, taking longer than neutrophils to reach infection sites.

• Lymphocytes: Key immune system soldiers. B lymphocytes produce antibodies, T lymphocytes are involved in cell-mediated destruction, and natural killer cells recognize and destroy diseased/infected cells.

Immunity

• The body's ability to resist or eliminate harmful foreign materials/abnormal cells. It includes defense mechanisms against microorganisms/pathogens, abnormal cells (cancer), or foreign cells.

Innate Immunity

• Defenses present from birth.
• External physical/chemical barriers (skin, mucous membranes).
• Internal nonspecific defenses (antimicrobial proteins, NK cells, phagocytes).
  • First line: external physical barriers like skin & mucous membranes, and chemical barriers like sebum, gastric juices, tears, saliva.
  • Second line: antimicrobial proteins (interferons, iron-binding proteins), and phagocytes (monocytes, neutrophils).

Adaptive Immunity

• Defenses developed after birth.
• Ability to recognize and target specific pathogens using antibodies.
• Involves T lymphocytes(T cells) and B lymphocytes(B cells).
• Has distinct cellular and humoral components.

Antibody-mediated Immunity

• Works against extracellular pathogens, using antibodies produced by B cells to target pathogens outside cells. -Types: Active (exposure to antigens, producing antibodies), and Passive (receiving antibodies from another source).

Immunoglobulins, also known as antibodies, are specialized glycoproteins produced by B lymphocytes in response to the presence of antigens, which are foreign substances that induce an immune response. There are five main classes of immunoglobulins in humans: IgA, IgD, IgE, IgG, and IgM, each with distinct functions and properties.

• IgA is predominantly found in mucous membranes lining the respiratory and gastrointestinal tracts, as well as in secretions such as saliva, tears, and breast milk, where it plays a crucial role in mucosal immunity.
• IgD is primarily found on the surface of immature B cells and is believed to play a role in B cell activation and development.
• IgE is associated with allergic responses and protection against parasitic infections. It binds to allergens and triggers histamine release from mast cells and basophils, leading to inflammation.
• IgG is the most abundant class of immunoglobulin in the serum and is critical for long-term immunity. It can neutralize toxins, opsonize pathogens for phagocytosis, and activate the complement system.
• IgM is the first antibody produced in response to an infection and is effective in agglutinating pathogens, facilitating their elimination.

Overall, immunoglobulins are vital components of the adaptive immune system, helping to identify and neutralize a wide range of pathogens and foreign substances. They also provide long-lasting immunity by forming immunological memory following infection or vaccination.

• IgG, IgM, IgA, IgE, and IgD are types of immunoglobulins having diverse roles in the immune system.

Inflammation

• A nonspecific defensive response of the body to tissue damage. It is protective to: eliminate the cause of injury, remove damaged tissue, generate new tissue. Signs/symptoms: redness (rubor), swelling (tumor), heat (calor), pain (dolor), loss of function (laesa).
• Stages: vascular permeability/vasodilation and mediator release, exudate production and formation, tissue repair and regeneration.

Fever

• A common response to infection and inflammation. Elevated body temperature enhances effects of interferon, which inhibits microbial growth; speeds up cellular repair processes.