BIOL 2304
Lymphatic and Immune Systems
The Lymphatic System
Functions:
Transport excess interstitial fluid back to bloodstream
Transport dietary lipids
Contain lymphocytes that filter and cleanse lymph
Generate an immune response
Components:
Lymphocytes, Lymph nodes, Lymphatic vessels
Lymphoid follicles in GI tract (Peyer's patches, tonsils)
Other accessory organs: spleen, thymus, bone marrow
Orders Of Lymphatic Vessels
Lymph capillaries – smallest lymph vessels, first to receive
lymph
Lymphatic collecting vessels – collect from lymph capillaries
Lymph nodes – scattered along collecting vessels
Lymph trunks – collect lymph from collecting vessels
Lymph ducts – empty into veins of the neck
Lymphatic Capillaries
Located near blood capillaries
Receive tissue fluid from CT
Minivalve flaps open and allow fluid to enter
Highly permeability and low pressure allows entrance of ECF, bacteria, viruses, and cancer cells
Lymphatic Capillaries
Lacteals – specialized lymphatic capillaries
Specialized lymphatic capillaries located in the villi of the small intestine
Receive large digested fats
Chyle – milky bodily fluid within lacteals made of a mixture of lymph and lipoproteins called
chylomicrons
Lymphatic Collecting Vessels
Accompany blood vessels and nerves as neurovascular bundle
Composed of the same three tunics as blood vessels: intima, media, externa
Contain more valves than veins do
Prevents backflow of lymph
Lymphatics have zero blood pressure
Lymph is propelled by:
Contraction of skeletal muscles
Pulse pressure of nearby arteries
Tunica media (smooth muscle) of the lymph vessels
Lymph Nodes
Cleanse the lymph of pathogens
~500 lymph nodes in human body
Lymph nodes are organized in clusters
Microscopic Anatomy of a Lymph Node
Fibrous capsule – surrounds lymph node
Trabeculae – connective tissue strands formed from capsule
Lymph vessels:
Afferent lymphatic vessels
Efferent lymphatic vessels
Lymph sinuses – regions within node containing network of reticular fibers on which immobile phagocytes and
lymphocytes await passing pathogens
Subcapsular, cortical, and medullary sinuses
Lymph Trunks
Lymphatic collecting vessels converge into trunks
Five major lymph trunk types:
Lumbar trunks (paired)
Receives lymph from lower limbs
Intestinal trunk (unpaired)
Receives chyle from digestive
organs
Bronchomediastinal trunks (paired)
Collects lymph from thoracic
viscera
Subclavian trunks (paired)
Receive lymph from upper limbs
and thoracic wall
Jugular trunks (paired)
Drain lymph from the head and neck
Lymph Ducts
Cisterna chyli – a dilated sac located at the union of lumbar and intestinal trunks
Thoracic duct – ascends along vertebral bodies
Drains 3/4th of the body
Empties into left brachiocephalic vein between the left subclavian and left internal jugular veins
Right lymphatic duct –
Drains 1/4th of the body
Empties into right internal jugular vein and right subclavian veins
The Immune System
Reconnaissance, Recognition, and Response
Components:
Immune cells, lymphoid tissue, and lymphoid organs
Functions of Immune Cells:
Detect invader/foreign cells
Communicate alarm & recruit immune cells
Suppress or destroy invader
Two major kinds of defense:
Innate immunity – inborn, no exposure to antigen required
Acquired immunity – develops after exposure to antigen
Review of Blood Cells
Erythrocytes (red blood cells)
Thrombocytes (platelets) – anuclear cell fragments; essential for blood clotting
Leukocytes (white blood cells)
Granulocytes – Presence of granules in their cytoplasm
Neutrophil: 50-70% of WBC; most common WBC; phagocytic; first response inflammation
Eosinophil: <5% of WBC; fight parasites
Basophil: <1% of WBC; release histamines (vasodilator) and heparin (anticoagulant)
Agranulocytes – (aka lymphoid cells) Absence of granules in their cytoplasm
Lymphocytes: 25-35% of WBC
T-lymphocytes (T cells): fight virus-infected cells and cancer cells
B-lymphocytes (B cells): produce & secrete antibodies
Natural Killer Cells (NK cells): fight virus-infected cells and
cancer cells
Monocytes: 3-9% of WBC; largest of the leukocytes
Leave the blood stream (diapedesis) to become
macrophages
Primary Lymphocytes
T Lymphocytes:
Helper T lymphocytes
Alert other leukocytes of presence of pathogens
Cytotoxic T lymphocytes
Attack foreign cells directly
B Lymphocytes:
Become plasma cells and secrete antibodies
Lymphocytes – B and T cells
Different Types of T cells
Cytotoxic T cells – attack foreign cells
Helper T cells – activate other T cells and B cells
Suppressor T cells – inhibit the activation of T and B cells
Memory T cells – dormant T cells that are activated at second exposure to antigen
T cell membranes contain CD markers:
CD3 markers present on all T cells
CD8 markers on cytotoxic and suppressor T cells
CD4 markers on helper T cells
Different Types of B cells
Plasma cells – activated B cells that secrete antibodies
Memory B cells – dormant B cells that are activated at second exposure to antigen
Lymphocyte Formation
B and T cells originate in bone marrow
B cells remain in bone marrow for maturation
T cells leave bone marrow, and migrate to thymus gland for
maturation
Lymphocyte Activation
All lymphocytes originate in bone marrow
B lymphocytes remain in bone marrow for maturation
T lymphocytes leave bone marrow, and migrate to
thymus gland for maturation
Lymphocytes provide Acquired Immunity:
Specificity – activated by and responds to a specific
antigen
Versatility – is ready to confront any antigen at any time
Memory – “remembers” any antigen it has encountered
Tolerance – responds to foreign substances but ignores
normal tissues
Immunocompetence – the body’s ability to produce a
normal immune response following exposure to an
antigen
Lymphocyte Activation
Activating T or B cells produce:
Effector lymphocytes – short-lived, active, attack immediately
Memory lymphocytes – dormant until body encounters same antigen again
Basis of acquired immunity
Guard against subsequent infections
Lymphoid Tissue
Lymphoid tissue – specialized connective tissue in which vast quantities of lymphocytes gather to fight invading
microorganiss
Two general types based on location:
Mucosa-associated lymphoid tissue (MALT) - mucous membranes of digestive, urinary,
respiratory, and reproductive tracts
Found within all lymphoid organs except thymus
Lymphoid follicles (nodules) –
Spherical clusters of densely packed lymphocytes; not surrounded by a fibrous capsule; found
scattered in lymphoid tissue
Lymphoid Organs
Primary lymphoid organs
Bone marrow
Thymus
Secondary lymphoid organs
Lymph nodes
Spleen
Tonsils
Aggregated lymphoid follicles (nodules) in small intestine and appendix
Thymus
Immature lymphocytes develop into T lymphocytes
Most active in childhood
Functional tissue atrophies with age
Composed of cortex and medulla
Medulla contains Hassall’s corpuscles (thymic corpuscles)
Differs from other lymphoid organs
Functions strictly in T lymphocyte maturation
Arises from epithelial tissue
Lymph nodes
Lymph nodes
Lymph percolates through lymph sinuses
Sinuses filled with network of reticular fibers containing phagocytes
Most antigenic challenges occur in lymph nodes
Spleen
Largest lymphoid organ
Two main blood-cleansing functions
Removal of blood-borne antigens
Removal and destruction of old or defective blood cells
Storage of platelets
White pulp
Thick sleeves of lymphoid tissue
Provides the immune function of the spleen
Red pulp - surrounds white pulp, composed of:
Venous sinuses – filled with whole blood
Splenic cords – reticular CT rich in macrophages
Also the site of hematopoiesis in the fetus
Tonsils
Simplest lymphoid organs
Four groups of tonsils
Palatine, lingual, pharyngeal and tubal tonsils
Arranged in a ring to gather and remove pathogens
Underlying lamina propria consists of MALT
Aggregated Lymphoid Nodules And Appendix
MALT – mucosa-associated lymphoid tissue
Abundant in walls of intestines
Fight invading bacteria
Generate a wide variety of memory lymphocytes
Aggregated lymphoid nodules (Peyer’s patches)
Located in the distal part of the small intestine
Appendix – tubular offshoot of the cecum