The Lymphatic System

• The lymphatic system's functions include immunity, interstitial fluid return, and fat transport from the small intestine to veins.

Immunity

• Transports WBCs and pathogens to lymph nodes.
• Facilitates antibody production.

Lymphatic Hierarchy

• Lymphatic capillaries collect interstitial fluid (lymph).
• Lymphatic vessels contain valves and periodic lymph nodes.
• Lymphatic trunks collect lymph from lymphatic vessels.
• Lymphatic ducts receive lymph from lymphatic trunks and empty into subclavian veins.

Lymphatic Capillaries

• Collect interstitial fluid (lymph).
• Have a similar anatomy to blood capillaries (simple squamous epithelium).
• Feature closed ends and are located at the same distance as blood capillaries.

Lymphatic Vessels

• Possess the same tunic anatomy as veins.
• Contain semi-lunar valves.
• Characterized by periodic lymph nodes (glands).

Lymphatic Trunks

• Collect lymph from lymphatic vessels.
• Share the same tunic anatomy as veins.

Lymphatic Ducts

• Collect lymph from lymphatic trunks.
• Both ducts empty into subclavian veins.
• Share same tunic anatomy as veins

Lymph Nodes (Glands)

• Destroy pathogens.
• House dividing and mature lymphocytes and monocytes (macrophages).
• Are situated along lymphatic pathways.
• Cervical nodes enlarge with throat or respiratory infections.
• Sentinel lymph nodes are the first to drain a tumor.

Elephantiasis (Lymphedema)

• Results from lymphatic node obstruction caused by nematode worms, tumors, or infection.
• Leads to tissue edema and collagen fiber formation.

Lymph Formation

• Blood capillary filtration forces plasma (minus proteins) into interstitial spaces.
• The fluid then enters lymph capillaries.

Lymph Flow

• Skeletal muscle contraction in limbs.
• Contraction of respiratory muscles and abdominal viscera.
• Smooth muscle contraction in larger lymphatic vessels and trunks.

Lymphatic Glands and Organs

• Lymph nodes.
• Thymus.
• Spleen.
• Tonsils.
• Peyer's patches.
• Appendix.

Thymus

• Facilitates the maturation of "T" lymphocytes from red marrow.
• Shrinks after puberty and is partially replaced by connective tissue in the elderly.

Spleen

• The body's largest lymph node.
• Filters blood similarly to how nodes filter lymph.
• Involved in hemopoiesis in the fetus.
• Destroys old RBCs via capillary channels, cellular debris via macrophages, and pathogens via lymphocytes and macrophages.
• Capable to fragment or shed stem cells and grow new organs (spleen) within the body.

Tonsils

• Are unencapsulated lymph tissue.
• Gather and remove inhaled and ingested pathogens.
• Bacteria are attacked by dense concentrations of lymphocytes.
• Tonsillitis is an inflammation of the tonsils (usually palatine), typically caused by bacteria.

Peyer's Patches

• Are aggregated lymphoid nodules in the ileum (small intestine).
• Gather and remove ingested pathogens.

Appendix

• Gathers and removes ingested pathogens.
• Appendicitis results from bacterial infection, leading to inflammation.

Lymphatic System's Response to Pathogens

• Non-specific defense.
• Specific defense.

Non-Specific Defense

• The defense mechanism remains the same regardless of the type of pathogen.
• Includes phagocytosis, fever, and inflammation.

Specific Defense

• Defense is directed to a specific antigen(s).
• Involves blood, antigens, and immunizations.

Non-Specific Defense Mechanisms

• Species resistance.
• Mechanical barriers.
• Chemical barriers.
• Fever.
• Inflammation.
• Phagocytosis.

Species Resistance

• An entire species resists a group of pathogens.
• Evolution of new pathogen variants can alter species resistance.

Mechanical Barriers

• Integument: hard to penetrate & sloughs off carrying pathogens
• Mucous membranes: carries pathogens in mucous to digestive system

Chemical Barriers

• Interferons: Interferes with virus replication in other cells
• Defensins: punch holes in bacteria membranes & cell walls
• Collectins: Detect subtle differences in non-self antigens
• Misc. Chemicals: HCI acid & Pepsin in stomach, salt on skin and antibacterial in tears.

Fever

• Lymphocyte secretion raises body temperature.
• Increased temperature causes the liver and spleen to retain iron.
• Bacteria and fungi are unable to reproduce without iron.
• Phagocytosis increases, and high temperatures denature pathogen proteins.

Inflammation

• Results in localized swelling, redness, and heat.
• Initiated by basophils.
• Is non-specific.

Phagocytosis

• Involves the removal of cells and debris by phagocytes (neutrophils, monocytes, and macrophages).
• Macrophages fix themselves to tissues and vessels.
• Phagocytes are attracted to chemicals released by damaged tissues.

Immunity

• Resistance to a particular pathogen or its toxins.
• Is the result of lymphocytes and macrophages recognizing "non-self/foreign-antigens".

Lymphocytes

• Inventory self-antigens prepartum.
• Possess receptors that recognize antigens.

"T" (Thymus-Derived) Cells

• Formed in red marrow, mature in the thymus, and reside in lymphatic organs/glands.
• Helper T cells mobilize the immune response by activating "B" cells.
• Memory T cells prepare the system for a rapid response to reinfection.
• Cytotoxic T cells destroy cancerous and virus-infected cells by destroying membranes.
• Natural killer T cells attack cancerous and virus-infected cells without requiring a foreign antigen for stimulus.

"B" Cells

• Formed and mature in red marrow and reside in lymphatic organs/glands.
• B plasma cells produce antibodies for defense.
• Memory B cells prepare the system for a rapid response to reinfection.

Immune Response to Foreign Antigens

• Macrophages engulf foreign cells, display foreign antigens, and travel to lymph nodes to activate Helper T's as antigen-presenting cells.
• Helper T's dock with macrophage-displayed foreign antigens and release cytokines that stimulate B cells to reproduce.
• B cells produce clones (plasma cells) that release antibodies that mimic B cell antigen receptors.
• Antibodies dock with foreign antigens and trigger the destruction of the cell/particle/virus.
• Some B and T cells turn into memory B and T cells.

Antibodies

• An individual B cell produces one type of antibody, although variant forms of B cells can produce other types.
• Pathogens have multiple forms of antigens, leading to a polyclonal immune response.
• Polyclonal immune response is when many antibodies work together to take down the pathogen.

Humoral Immunity (Antibody-Mediated Immunity)

• Is the production of antibodies to fight pathogens.

Cell-Mediated Immunity

• Is the destruction of pathogens by direct cellular activity (cytotoxic and killer T cells).

Humoral Immune Response

• Primary: activation of immune response upon initial exposure to a particular pathogen.
  • Antibodies produced in 5-10 days and last several months or years.
• Secondary: activation of immune response upon repeat exposure to the same pathogen, governed by T & B cells.
  • Antibodies produced take 1-2 days, are higher in amount, and memory cells live for many years.
• Would rather have a secondary response because its quick and produces a higher number of antibodies