Histo Lecture 16 Lymphoid Organs

Front 1

Primary Lymphoid organs

Back 1

Thymus
Bone Marrow

Front 2

Secondary Lymphoid Organs

Back 2

Lymph nodes
Spleen
MALT (mucosa-associated lymphoid tissue)
-tonsils
-peyer's patches of the ileum
-lymphoid follicles of the GI, resp. + GIU tracts (GI + urinary)
-Diffuse populations of lymphoid cells in the mucosa

Front 3

Sources of lymphocytes

Back 3

Bone marrow: T-cell
Thymus

Front 4

Features of a thymus in a child

Back 4

Lobe, Lobulues
Capsule
Septa
Cortex (outer and inner cortex)
Corticomedullary junction
Medulla (common to/communicates with all lobules - lighter staining)
NO AFFERENT LYMPHATICS
There are efferent lymphatics draining the capsule and CT septa

Front 5

Functions of the thymus

Back 5

-provides an environment where T cell precursors proliferate, mature, and acquire their immunocompetence
-supplies mature T cells to secondary lymphoid organs
-secretes hormones which stimulate T cell proliferation and maturation

Front 6

Outer cortex of the thymus: major cell types

Back 6

Immature T cells (large lymphocytes of lymphoblasts)
Maturing T cells (small lymphocytes)
Epithelial reticular cells - more elongated cell/nuclei, lighter staining
Macrophages - larger cell, more spherical

Front 7

Where do lymphoblasts proliferate?

Back 7

Outer cortex of the thymus

Front 8

Where does elimination of self-reactive T cells occur? What percentage are eliminated?

Back 8

Inner cortex of the thymus, where 95% are eliminated!

Front 9

How are self-reactive T cells eliminated?

Back 9

They die by apoptosis, and are phagocytized by macrophages, in a process known as clonal deletion or negative selection. This prevents the development of autoimmunity.

Front 10

How do mature non-reactive T cells move to the corticomedullary junction?

Back 10

Via the post capillary venules

Front 11

List the Thymic hormone-like factors?

Back 11

Thymulin
Thymosin
Thymopoietin

-polypeptides produced by the epithelial reticular cells
-regulate T cell proliferation and maturation within the thymus and in secondary lymphoid organs and tissues

Front 12

Hassall's corpuscle

Back 12

Histological marker for the thymus
First appear in fetal life and increase in number and size thereafter
No other lymphoid tissue has these
Don't serve any known role in T-cell maturation

Front 13

How to plasma cells and IgA prevent attachment of bacteria/toxins to enterocytes?

Back 13

B-lymphocytes --> Plasma cells --> IgA --> binds antigens

Front 14

What are primary lymphoid follicles/nodules?

Back 14

Ovoid structures containing aggregates of lymphocytes-- B-lymphocytes.

Front 15

What are secondary lymphoid follicles/nodules?

Back 15

Follicles containing a central pale area = germinal center, site of B-cell activation.

Front 16

What type of follicles are in Peyer's patches? What about Pataline tonsils?

Back 16

Peyer's patches of the ileum contain large aggregates of SECONDARY follices.

Palatine tonsil contains secondary follices with germinal centers.

Front 17

What are M cells?

Back 17

"microfold" cells - epithelial cells (with surface microfolds) overlying the Peyer's patches.
Important antigen-presenting cell. Presents antigen to macrophages --> macrophages present to T-cells, etc...

Front 18

Where don't you find lymph nodes/lymphatic vessels?

Back 18

in the CNS: NO LYMPH NODES, NO LYMPHATIC VESSELS

Front 19

What are the parts of a lymph node?

Back 19

Hilus
Medulla
Cortex: Superficial, Deep/Paracortex
(lymphocytes: T-cell, B-cell)
Capsule

Front 20

Describe the capsule of lymph nodes. What collagen type?

Back 20

Sends septum (trabeculae) into lymph node, made of Type I collagen

Front 21

What type of collagen is in the Parenchyma of the lymph nodes?

Back 21

Reticular fibers (type III collagen)

Front 22

What are the functions of the lymph node?

Back 22

-filtration of particulate matter, microorganisms, and antigens from the lymph
-Presentation of antigens to lymphocytes by macrophages and APC's
-Activation and proliferation of B cells, leading to plasma cell formation and antibody production
-Activation and proliferation of T cells

Front 23

What is the pathway by which lymph flows through a lymph node, i.e. the "Filtration Pathway"?

Back 23

Afferent lymphatic vessels --> subcapsular sinus under the capsule--> trabecular sinuses on each side of trabeculae --> medullary sinuses --> efferent lymphatic vessel

Front 24

Do lymphatic vessels have valves?

Back 24

YES. V-A-L-V-E. VALVES.

Front 25

Describe lymphocyte traffic through lymph nodes

Back 25

Lymphocytes enter via artery, afferent lymphatic vessels, post-capillary venules
Lymphocytes exit via efferent lymphatic vessels --> thoracic duct --> veins --> heart

Front 26

What are High Endothelial Venule?

Back 26

Post-capillary venules found in paracortical region of the lymph nodes, made of cuboidal cells (not squamous!), express adhesion molecules (addressins) that function in "lymphocyte homing"

Front 27

What are the different lymphocyte domains in the lymph nodes?

Back 27

B cells in follicles of superficial cortex

T cells in paracortex

Plasma cells and B cells in medullary cords?

Front 28

What are different structural features of spleen tissue?

Back 28

Red pulp
White pulp
Capsule; trabeculae

Front 29

What types of collagen are in different structures in spleen tissue?

Back 29

Capsule & trabeculae: Collagen I
Prenchyma: Reticular fibers (type III)

Front 30

What are the functions of the spleen?

Back 30

Immunity in response to blood borne antigens
White pulp: Production of activated B-cells and antibodies
Production of T cells
Red pulp: Removal of aged and defective RBC from the blood circulation.

Front 31

What are structural features of the white pulp of the spleen?

Back 31

Central artery
PALS = periarterial lymphoid sheath (T cell domain)
Germinal center (B-cell domain)

Front 32

What are structural features of the red pulp of the spleen?

Back 32

Venous sinuses that contain blood
Splenic cords (Bilroth's cords)

Front 33

What are found in Bilroth's cords?

Back 33

Bilroth's cords (splenic cords of red pulp) contain reticular cells, macrophages, T-cels, B-cells, plasma cells, granulocytes, platelets, and RBCS

Front 34

Describe the structure of splenic venous sinuses

Back 34

Basement and membranes and reticular fibers wrap around a "barrel" of endothelial cells, creating a tunnel with a small lumen. Aged and defective RBCs cannot pass through the endothelium of the venous sinus, and are eaten by macrophages.