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Anne ([email protected])
Tresha ([email protected])
Lymphoid System I: Peripheral System, Lymph Node
To put this in context:
Your acquired immune system is made of B and T cells, each specific for a single antigen. B and T cells are born in the bone marrow
and then matured in the primary lymph organs (bone marrow and thymus respectively). They then circulate in the blood as naïve
lymphocytes. Having never “seen” their antigen, they leave the blood through high endothelial venules (HEV) to filter through
secondary lymph organs – lymph nodes, Peyer’s patches, and the spleen – in hopes of finding their perfect match. Antigen is
concentrated in these 2o lymph organs by several mechanisms including afferent lymphatic vessels, M cells, and antigen presenting
cells (APC’s). APC’s act as sentinels, scouting for antigen in the skin and mucosa. When they find antigen, they eat it and quickly
travel with the lymph via lymphatic vessels to the nearest lymph node. In the node, lucky and specific B cell and T cells interact with
the antigen in follicles, proliferating and maturing. Mature B and T cells then exit via the blood vessels to fight off the infection.
Mature B cells may also stay in the lymph node as plasma cells, secreting their major protein product, antibody.
Lymph Vessels:
-More permeable then blood vessels
-Drain extracellular space, delivering free antigen and cells (APC’s) to lymph nodes, connecting nodes.
-Valves prevent backflow
-Thinner, less obvious than veins
Lymph Follicle:
Site of antigen encounter and proliferation/differentiation of B and T cells.
Primary Follicle:
Secondary follicle
-No germinal center
-Antigen has been encountered
-Small lymphocytes, naïve
-Germinal center = site of B cell proliferation!
Stains pale b/c of many plasmablasts with large,
euchromatic nuclei and prominent nucleoli.
Some plasmablasts will be in mitosis.
-T cells are helping at the periphery
B cell
Lymphoid Diffuse Lymphoid Tissue:
GALT, S/B/MALT, tonsils, appendix, etc.
-Found in the lamina propria of GI tract, respiratory
tract, GU tract, where pathogens are likely to invade.
-No capsule
Peyer’s patches in gut
-Antigen enters via transcytosis across special M cells in
the epithelium. NO afferent lymphatics.
Tonsil: ring of diffuse lymphatics in oropharynx
-Crypts of stratified squamous non-keratinized
-Lymphocytes can be seen invading epithelium
Anne ([email protected])
Tresha ([email protected])
Encapsulated Lympoid Tissue:
Spleen, Lymph nodes:
Lymph flow w/i node:
Afferent lymphatics  subcapular sinus  trabecular
sinuses  medullary sinuses  efferent lymphatics
Lymph Node:
-Capsule: Dense Irregular CT
Trabeculae are extensions of the capsule
-Subcapsular, trabecular space
Macrophages (visualized with India ink)
Outermost area = follicles (B cell)
Deep cortex = T-zone
- post-capillary venules (“high”
endothelial venules = cuboidal or
Tcells enter & exit blood stream here!
Made of cords and sinuses
Plasma cells (mature and darker staining B
cells) secrete antibodies
-Hilum: indented area surrounded by the medulla
CT, fat, blood vessels (afferent and efferent),
nerves, efferent lymphatics with valves.
Bielschowsky silver stain
*Capsule is DICT (collagen I stains red-brown)
*Stroma is reticular tissue (collagen III stains brownblack)
Anne ([email protected])
Tresha ([email protected])
Q1) For the two pictures on the slide, select the correct
answer for each of the below:
Is encapsulated
Contains germinal centers
Peripheral lymphoid tissue
Central lymphoid tissue
Q2) The region at the arrow is:
Peyer’s Patch
Germinal Center
Lymph node
Primary lymphoid nodule
Q3) The region inside the box contains which of the
following upon antigenic stimulation:
Plasma Cells
Memory B-Cells
All of the Above
A = tonsil Lab 11, Slide 2
B = lymph node Lab 11, Slide 9
C (Both)
C (Both)
D (Neither) The thymus and bone marrow
are the central lymphoid tissues
Q2) D Lab 11, Slide 4
This lymphatic tissue in the gut is formed into a
lymphatic nodule, but there is no germinal center so it is
primary. It is not a Peyer’s patch because that is a group
of lymph nodules in the GI tract.
Q3) E. Plasma cells are secreting antibody into efferent
lymph. Macrophages continue to filter the medullary
sinuses with the efferent lymph. Memory T and B-cells
exit the node via the efferent lymph node.