Download Lymphoid System I: Peripheral System, Lymph Node

Lymphoid System I: Peripheral System, Lymph Node
Sarah Little: sjl2017
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 bvv
Lymph Follicle:
Site of antigen encounter and proliferation/differentiation of B and T cells.
Secondary follicle
Primary Follicle:
-Antigen has been encountered
-No germinal center
-Germinal center = site of B cell proliferation!
-Small lymphocytes, naïve
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:
Encapsulated Lympoid 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
Spleen, Lymph nodes:
GALT:
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
epithelium
-Lymphocytes can be seen invading epithelium
Lymph flow w/i node:
Afferent lymphatics Æ subcapular sinus Æ
trabecular sinuses Æ medullary sinuses Æ
efferent lymphatics
Lymph Node:
-Capsule: Dense Irregular CT
Trabeculae are extensions of the capsule
-Subcapsular, trabecular space
Macrophages (visualized with India ink)
-Cortex:
Outermost area = follicles (B cell)
Deep cortex = T-zone
- post-capillary venules (“high”
endothelial venules = cuboidal or
columnar)
Tcells enter & exit blood stream here!
-Medulla
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
brown-black)