Lymphatic and Immune Systems

Immunity
fluids from all capillary beds are filtered
immune cells stand ready to respond to foreign cells or chemicals encountered
Lipid absorption
Lacteals in small intestine absorb dietary lipids
Fluid recovery
absorbs plasma proteins and fluid (2 to 4 L/day) from tissues and returns it to the bloodstream
interference with lymphatic drainage leads to severe edema
Functions of Lymphatic System

Route of Lymph Flow
Lymphatic capillaries
Collecting vessels: course through many lymph nodes
Lymphatic trunks: drain major portions of body
Collecting ducts :
right lymphatic duct – receives lymph from R arm, R side of head and thorax; empties into R subclavian vein
thoracic duct - larger and longer, begins as a prominent sac in abdomen called the cisterna chyli, receives lymph from below diaphragm, left arm, left side of head, neck and thorax; empties into L subclavian vein

Mechanisms of Lymph Flow
Lymph flows at low pressure and speed
Moved along by rhythmic contractions of lymphatic vessels-stretching of vessels stimulates contraction
Flow aided by skeletal muscle pump
Thoracic pump aids flow from abdominal to thoracic cavity
Valves prevent backward flow
Rapidly flowing bloodstream in subclavian veins, draws lymph into it
Exercise significantly increases lymphatic return

Lymphatic Cells
T lymphocytes
Mature in thymus
B lymphocytes
Activation causes proliferation and differentiation into plasma cells that produce antibodies
Antigen Presenting Cells
Macrophages (from monocytes)
dendritic cells (in epidermis, mucous membranes and lymphatic organs)
reticular cells (also contribute to stroma of lymph organs)

Lymphatic Tissue
Diffuse lymphatic tissue: lymphocytes in mucous membranes and CT of many organs
Mucosa-Associated Lymphatic Tissue: particularly prevalent in passages open to the exterior
Lymphatic nodules: dense oval masses of lymphocytes, congregate in response to pathogens
Peyer patches: more permanent congregation, clusters found at junction of small to large intestine

Lymphatic Organs
At well defined anatomical sites, have CT capsules
Lymph nodes
cervical, axillary and inguinal regions close to surface
thoracic, abdominal and pelvic groups deep in cavities
Tonsils
guard entrance to pharynx
Thymus
between sternum and aortic arch
Spleen
inferior to diaphragm, dorsolateral to stomach

Lymph Node

Defenses Against Pathogens
Nonspecific defenses - broadly effective, no prior exposure
external barriers
phagocytic cells, antimicrobial proteins, inflammation and fever
Specific defense - results from prior exposure, protects against only a particular pathogen
immune system

External Barriers
Skin
toughness of keratin
dry and nutrient-poor
defensins: peptides, from neutrophils attack microbes
lactic acid (acid mantle) is a component of perspiration
Mucous membranes
stickiness of mucus
lysozyme: enzyme destroys bacterial cell walls
Subepithelial areolar tissue
tissue gel: viscous barrier of hyaluronic acid
hyaluronidase: enzyme used by pathogens to spread

Leukocytes and Cutaneous Defenses
Neutrophils
phagocytize bacteria
create a killing zone
degranulation: lysosomes discharge into tissue fluid, triggers
respiratory burst: toxic chemicals are created (O2.-, H2O2, HClO)
Eosinophils
phagocytize antigen-antibody complexes, allergens, inflammatory chemicals
antiparasitic effects: aggregate and release enzymes

Other Leukocytes
Basophils
aid mobility and action of WBC’s by the release of
histamine (vasodilator) ↑ blood flow to infected tissue
heparin (anticoagulant) prevents immobilization of phagocytes
Monocytes
circulating precursors to macrophages
Lymphocytes
natural killer (NK) cells, nonspecific defense, large cells lyse host cells infected with viruses or cancerous
by release of perforin proteins

Antimicrobial Proteins
Interferons: polypeptides secreted by cells invaded by viruses
antiviral effect
generalized protection
interferons diffuse to neighboring cells and stimulate them to produce antiviral proteins
activate natural killer cells and macrophages
destroy infected host cells
anticancer effect
stimulate destruction of cancer cells