Lymphatic System

  • Lymph nodes

    • oval nodules that cleanse the lymph


  • Lymph

    •  fluid in the lymph vessels

  • Lymph Vessels (lymphatics)

    • Collect the excess protein-containing interstitial fluid and return it to the blood

      • Also transport pathogens to lymph nodes and absorbed fats from intestine to blood

    • Run along arteries and veins

    • Defense mechanism

    • More permeable than capillaries (Bigger spaces)

      • None in bone or teeth

  • Hydrostatic 

    • Outward pressure

  • osmotic colloid pressure

  • Inward pressure

Lymphatic Capillaries

Blind-ended vessels that weave between tissue cells and blood capillaries

  • Astrocytes 

    • extend like arms to connect lymphatics in meninges

    • Helps drain ECF (interstitial and CSF) and their waste

    • Malfunction may play role in degenerative diseases (e.g., Alzheimer’s


Similar to blood capillaries, but much more permeable due to two unique

endothelial cell modifications:

  • More permeable

    • Mini valve 

    • Anchored to matrix by collagen so increases ISF volume open minivalves

    • pressure rises inside lymphatic capillaries, minivalve flaps forced shut

      • prevents lymph from leaking back out as pressure

  • Lacteals-

    • In the small intestine, called lacteals, are specialized set of lymphatic capillaries for absorption and transport of digested fat

      • Fatty lymph (chyle) joins rest of lymph, eventually entering blood


Lymph goes through veins and eventually to right atrium

Has the tunicas like in arteries and veins

Has a network (anastomosis)

Superficial and deep

 Major Trunks

  • lumbar, bronchomediastinal, subclavian, and jugular trunks 

    • Paired

  • Intestinal trunk


  • Lymph is delivered from the nine trunks into one of the two large lymphatic ducts

    • Right lymphatic duct drains right upper limb and right side of head and thorax

  •  Larger thoracic duct drains rest of body

    •  Empties into venous circulation at junction of left internal jugular and left subclavian veins

    • In about half of individuals, starts out as enlarged sac called cisterna chyli


  • Cisterna 

    • little octopus 

    • Looks like a pouch with vessels coming off

    • Below IVC and right of aorta




Need skeletal muscle to move, breathing pressure, pulse pressure, and smooth muscles help move lymph along

The lymphatic system

Lot of nodes


Clinical 

  • Like larger blood vessels, larger lymphatics receive bloodsupply from a vasa vasorum

    • – Become congested with blood during severe inflammation, called lymphangitis

    • – Inflamed lymphatics appear as red streaks that are tender to the touch


  • Lymphangitis: 

    • Become congested with blood during severe inflammation

      • Need skeletal muscle to move, breathing pressure, pulse pressure, and smooth muscles help move lymph along


  •  Lymphedema: insufficient return of lymph to blood resulting in severe localized edema

    • – May be caused by tumors blocking lymphatics or removal of lymphatics during cancer surgery

    • – May improve if some lymphatic pathways remain and Enlarge


Lymphoid Cells

  • Agranulocytes

  • T lymphocytes

    • Activated T cells manage immune response; some directly attack and destroy infected cells

    • Mature in thymus

    • Direct attack

    • Attaches to another cell to destroy


  • B lymphocytes

    • Mature in lymph

    • Activated B cells produce plasma cells, which secrete antibodies that mark antigens for destruction by phagocytosis or other mean

    • Makes antibodies

  • Macrophages

    • phagocytize foreign substances and activate T cells

  • Dendritic cells

    • capture and deliver antigens to lymph nodes to activate T cells

      • Not whole pathogen required

      • Just antigen is needed

        • Like a wanted sign 

      • Shows to young cells to know what to target and activate


  • Reticular cells

    • fibroblast-like cells that produce reticular fiber stroma in lymphoid organs and tissues

    • Stroma 

      • (network that supports other cell types) 

      • (stroma=network)

    • Protein fibers that make up structure of lymphatic tissue 


Lymphoid Tissue

  • Diffuse lymphoid tissue

    • loose arrangement of lymphoid cells and reticular fibers

      • Found in virtually every body organ

      • Larger collections in lamina propria of mucous membranes


Lymphoid Organ

Bone marrow (B cells) and thymus (T-cells) are primary

  • Lymphoid organs are grouped into two functional categories:

    • Primary lymphoid organs: where T and B cells mature

      • B cells mature in red bone marrow

      • T cells mature in thymus

    •  Secondary lymphoid organs: where mature lymphocytes first encounter their antigens and become activated; include:

      • Lymph nodes and spleen

      • Collections of mucosa-associated lymphoid tissue (M ALT)that form:

        • Tonsils, Peyer’s patches in small intestine, and appendix

      • Diffuse lymphoid tissues


Lymph Nodes Cleanse Lymph and House Lymphocytes

  • Two basic protective functions:

    • cleansing the lymph: act as lymph “filters” as lymph is transported back to blood


  • Immune system activation: strategically located in sites where lymphocytes encounter antigens and are activated to mount attack against them

    • Many antigens brought to lymph nodes from their point of entry by dendritic cells, which present them to (and activate) T cells

Structure of Lymph Nodes

  • Medulla inner

    • Thin inward extensions of cortical tissue

      • Contains B cells, T cells, and plasma cells

      • Plasma cell is a type of b cell

  • Cortex outer

    • Superficial part contains densely packed follicles, many with germinal centers filled with dividing B cells








Has sinuses with macrophages to capture anything

Circulation in the Lymph Nodes

  • Afferent

    • Vessels going in 

  • Efferent

    • Vessels Exit



  • Buboes: infected lymph nodes (often pus-filled); bubonic plague named for them

    • Overwhelmed by bacteria they are trying to destroy

    • – Results in inflamed, swollen, and tender nodes—a condition often referred to (erroneously) as swollen “glands”—but correctly called lymphadenopathy


  •  Lymph nodes can become secondary cancer sites if metastasizing cells become trapped

    • Cancer-infiltrated lymph nodes are swollen but usually not painful, which helps distinguish cancerous nodes from infected.



Secondary Organs

Spleen

  • soft, blood-rich organ about size of fist

  • Largest lymphoid organ; located in left side of abdominal cavity, under diaphragm

    • Served by splenic artery and vein, which enter and exit at the hilum

    • Provides site for lymphocyte proliferation and immune surveillance and response

    • Cleanses blood of aged and defective blood cells and platelets (via macrophages)

      • Stores Iron, Monocytes

      • Recycles broken down RBC

      • Fetal EPO production


Spleen Removes bloodborne pathogens and aged red blood cells


  • Histology of Spleen

    • White pulp: where immune function occurs

      • Mostly lymphocytes on reticular fibers, forming “cuffs” around central arteries; appear as islands of white in sea of red

    • Red pulp: where old blood cells and bloodborne pathogens are destroyed

      •  Lots of RBCs and macrophages that engulf them

      •  Composed of splenic cords (reticular tissue) that separate blood-filled splenic sinusoids (venous sinuses)

MALT Guards the Body’s Entryways Against Pathogens

  • Mucosa-associated lymphoid tissue (MALT)

    • Lymphoid tissues in mucous membranes throughout body

      •  Lining the respiratory, genitourinary, and digestive tracts 

      •  Protects us from pathogens trying to enter body


Collections include:

  • Tonsils

    • Paired palatine tonsils: on either side at posterior end of oral cavity

      • Largest and most often infected

    • Lingual tonsil: lumpy collection of lymphoid follicles at base of tongue

    • Pharyngeal tonsil (adenoid if enlarged): 

      • in posterior wall of nasopharynx

    • Pair of tubal tonsils

      • surround openings of auditory tubes into pharynx

    • Contain follicles with obvious germinal centers and diffusely scattered lymphocytes

  • On exam may ask which tonsil is which


  • Peyer’s patches

(aggregated lymphoid nodules): 

  • large clusters of lymphoid follicles in wall of distal portion of small intestine

    • Structurally like tonsils

    • Ideally located to:

      •  Prevent bacteria from breaching intestinal wall

      •  Generate “memory” lymphocytes for long-term immunity

  • Appendix

    • tubular offshoot of first part of large intestine that contains large number of lymphoid follicles

    • Like Peyer’s patches, ideally located to:

      • Prevent bacteria from breaching intestinal wall

      • Generate “memory” lymphocytes for long-term immunity


T Lymphocytes Mature in the Thymus

  • Thymus: bilobed lymphoid organ found in inferior neck; extends into mediastinum and partially overlies heart

Thymus comes from pharynx 

  • Atrophies during puberty

    • older adults, replaced almost entirely by fibrous and fatty tissue

  • Cortex contains rapidly dividing lymphocytes (the bulk of thymic cells) and scattered macrophages

  • Medulla contains fewer lymphocytes and structures called thymic corpuscles (concentric whorls of keratinized epithelial cells)

  • Regulatory T cells

    • important for preventing autoimmune response

Thymus has no B cells

  • Blood thymus barrier

    • keeps immature T lymphocytes isolated from antigens to prevent premature activation