Lymph I:
The Peripheral Lymph
System
Peripheral = Secondary
• Primary Immune Organs = bone marrow, thymus
– Site of maturation of cells of the immune system
• Secondary Immune Organs = Nodes, MALT,
spleen
– Filter blood and lymph
– Site of antigen, immune system interaction
Lymph Vessels
• Valves
• Carries Lymph from
tissue Æ lymph nodes
Æblood via thoracic duct
–
–
–
–
–
Fluid
Proteins
Ingested fats
Particulate matter
Antigen
Lab 11 Slide 7
Peripheral Lymph Organs
• Diffuse
– Eg. Peyer’s patches, tonsils, appendix
– Non-encapsulated
– In GI, respiratory, GU, and tracts, tonsils, appendix
• Encapsulated
– Spleen, lymph nodes
The Follicle
Primary
- Lymphocytes
Secondary
- Germinal Center
- Looks pale!
- Antigen has been
encountered
Lab 11 Slide 9
Secondary Follicle
Germinal Center
• Pale staining center:
Plasmablasts = B cells
– Large, euchromatic nuclei
– Prominent nucleoli
– Mitotic activity
• Dark periphery
– T cells helping out
Lab 11 Slide 15
Diffuse Lymphoid Tissue
• At most likely site of
infection
• GALT = “Gut Associated
Lymphatic Tissue”
– Prototype for diffuse
lymphatics: S/B/MALT
Lab 11 Slide 4
GALT
• “Peyer’s patches”
• In lamina propria of GI
• No afferent lymphatics!
– Antigen enters via
transcytosis across M
cells
Lab 11 Slide 4
Tonsils
Diffuse Lymph organs!
Not encapsulated!
You may see stratified,
squamous nonkeratinized
epithelium.
Lab 11 Slide 1
Encapsulated Lymphoid Tissue
• Eg. Spleen, Lymph
nodes
• Both afferent and
Efferent lymphatics
Lab 11 Slide 5
Lymph Node Architecture
•
•
•
•
•
Capsule: DICT (collagen I is red/brown in silver
stain) with trabeculae that extend into stroma of
node (collagen III is black/brown)
Subcapular space with macrophages (visualized
with India ink)
Cortex
– Germinal centers w/ B cells
– Paracortex around follicles
– Deep cortex w/ high endothelial venules
– Juxtamedullary cortex
Medulla w/ Plasma cells and Macrophages
– Cords
– Sinuses
Hilum
– Efferent lymphatics
– Afferent and efferent bvv
Lab 11 Slide 14
Lymph Node Architecture
•
•
•
•
•
Capsule: DICT (collagen I is red/brown in silver
stain) with trabeculae that extend into stroma of
node (collagen III is black/brown)
Subcapular space with macrophages (visualized
with India ink)
Cortex
– Germinal centers w/ B cells
– Paracortex around follicles
– Deep cortex w/ high endothelial venules
– Juxtamedullary cortex
Medulla w/ Plasma cells and Macrophages
– Cords
– Sinuses
Hilum
– Efferent lymphatics
– Afferent and efferent bvv
Lab 11 Slide 6
Lymph Node Architecture
•
•
•
•
•
Capsule: DICT (collagen I is red/brown in silver
stain) with trabeculae that extend into stroma of
node (collagen III is black/brown)
Subcapular space with macrophages (visualized
with India ink)
Cortex
– Germinal centers w/ B cells
– Paracortex around follicles
– Deep cortex w/ high endothelial venules
– Juxtamedullary cortex
Medulla w/ Plasma cells and Macrophages
– Cords
– Sinuses
Hilum
– Efferent lymphatics
– Afferent and efferent bvv
Lab 11 Slide 12
Lymph Node Flow
Afferent Lymphatics Æ Subcapsular sinus Æ trabecular
sinuses Æ medullary sinus Æ efferent lymphatics
Lab 11 Slide 5
Lab 11 Slide 14
Thymus and Spleen
Are you getting immune to exam
blocks yet??
Thymus=“Education of T cells”
• Thymocytes mature
and proliferate in the
thymus
• Thymocytes undergo
positive and negative
selection
• Only T cells that react
weakly with self
proteins survive
Thymus has 3 cell types
• Epithelioreticular cells
are large, pale, and
stellate (they are not
reticular fibers!)
• Thymocytes are
immature T cells
• Macrophages
phagocytose T cells
that react too strongly
with self
Thymocytes
Epithelioreticular cells
The Cortex and Medulla
• The cortex stains
darkly basophilic
because there lots of
small lymphocytes
with intensely stained
nuclei
• The medulla stains
light because it has
less lymphocytes with
more cytoplasm
Medulla
Cortex
Blood-Thymic Barrier
• Separates developing T
cells from blood (prevents
T cells from recognizing
foreign proteins as “self”)
• Components (from
outside)
– Capillary endothelium
– Endothelial basal lamina
– Perivascular connective
tissue sheath
– Basal lamina of
epithelioreticular cell
– Epithelioreticular sheath
Hassall’s corpuscles
• Hassall’s Corpuscles
instantly signify the
thymus!!
• They are concentrically
arranged keratinizing and
degenerating
epitelioreticular cells and
macrophages
• They are found in the
medulla
The Adult Thymus
• Adult thymus shrinks
• Adipose tissue
replaces thymic tissue
• Medulla and cortex
are harder to
differentiate because
there are less
lymphocytes
The Spleen: What is it good for?
1. Filters blood
2. Iron Retrieval
3. RBC reserve
4. Immune Response*
5. Fetal Hematopoiesis
The ‘White’ Pulp
• Appears basophilic on
H&E and red on silver
stain
• Site where immune
response is mounted;
formation of germinal
centers
• Germinal centers with B
cells and B cell
derivatives push the
‘central artery’ off to the
side
White Pulp Vasculature
• The central artery is
found in the white
pulp
• The central artery is
surrounded by the
PALS, which is T
cells
• Penicilli branch from
the central artery into
the red pulp
Red Pulp Vasculature:
• Penicilli give rise to
ellipsoids
• Ellipsoids are capillaries
ensheathed by reticular
cells and macrophages;
their lumens are often
occluded in histo sections
• Blood is filtered by
macrophages through
fenestrations in the
sinusoids
Sinusoids
See how the basal lamina is interrupted; evident with
both stains
The Red Pulp
• Appears Red on H&E
• Composed of sinusoids
and Cords of Billroth
• The cords are the
parenchyma of the red
pulp; they are composed
of reticular tissue w/
macrophages, red blood
cells, and lymphocytes
Silver Stain
*Notice how reticular fibers are evident with silver stain and not H&E
*Notice the difference in appearance with the two different stains
That’s all folks
• We hope you got a good education
(THYMUS) & that you filtered out
what was important (SPLEEN)