BIOL 2304
Lymphatic and Immune Systems
The Lymphatic System
Functions:
Transport excess interstitial fluid (ISF) back to
the CVS
Transport dietary lipids from GI tract to the CVS
Contain lymphocytes that filter and cleanse
lymph
Help generate an immune response
Components:
Lymphocytes, Lymph nodes, Lymphatic vessels
Lymphoid follicles (Peyer's patches, tonsils)
Other accessory organs: spleen, thymus, bone
marrow
Types of Lymphatic Vessels
Lymph capillaries – smallest lymph vessels; first to
receive lymph from ISF
Lymphatic collecting vessels – collect from lymph
capillaries
Lymph nodes – scattered along collecting vessels; house lymphocytes that filter lymph (may become
inflamed/swollen during systemic infection)
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 loose CT
Minivalve flaps open and allow fluid to enter
Highly permeability and low pressure allows entrance of ISF, bacteria, viruses, and cancer cells
Lymphatic Capillaries
Lacteals – 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
chylomicronss
Lacteals
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
Valves prevent 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
Lymphoid Tissue
Lymphoid tissue – specialized CT in which vast quantities of lymphocytes gather to fight invading
microorganisms
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
Lymph nodes
Spleen
Tonsils
Aggregated lymphoid follicles (nodules) in small intestine and appendix
Bone marrow
Thymus
Thymus
Immature lymphocytes travel from bone marrow to thymus to mature into T lymphocytes
Lymph Nodes
Lymph percolates through lymph sinuses
Sinuses filled with network of reticular fibers containing phagocytes
Most antigenic challenges occur in lymph nodes
Spleen
The largest lymphoid organ
Two main blood-cleansing functions
Removal of blood-borne antigens
Removal and destruction of old or defective blood cells
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
Tonsils
Simplest lymphoid organs
Four groups of tonsils:
Pharyngeal tonsil (single)
At roof of the nasopharynx
Tubal tonsils (paired)
At opening of the Eustachian tube into the nasopharynx
Not visible in midsagittal sections of head
Palatine tonsils (paired)
At sides of oropharynx
Lingual tonsil (single)
On dorsal surface at the base of the tongue
Tonsils altogether arranged in a ring to gather and remove inhaled and ingested pathogens
Underlying lamina propria consists of MALT
sAggregated 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
(ileum portion only)
Appendix – tubular offshoot of the cecum
Overview