H & E: 3:00 - 4:00 Scribe: Caitlin Cox
Tuesday, October 20, 2009 Proof: Marjorie O’Neil
Dr. Bebok Immune System #2 Page 5 of 5
APCs – antigen presenting cells; CT – connective tissue; Ag – antigen; HEV – high endothelial venules;
RBCs – red blood cells; PALS – periarterial lymphatic sheath/system
I. Immune System #2 [S1]:
a. Skipped.
II. Role of the primary lymphoid organs [S2]:
a. This is the most important sentence to remember about the previous lecture: primary lymphoid organs provide sites for lymphocyte maturation where they become antigenically committed in the absence of the antigens.
III. Mature B and T cells [S3]:
a. Mature B and T cells will carry surface receptors depending on what type of cell they are.
b. They are antigenically committed.
c. They are small lymphocytes that you cannot distinguish by simple staining.
d. They are carried by blood or lymph to the secondary lymphoid organs.
IV. Secondary Lymphoid Organs [S4]:
a. The secondary lymphoid organs are scattered throughout the body, which is why they can provide systemic protection against infectious agents and foreign substances.
b. Lymph nodes are scattered throughout the body. They are present:
i. in different regions of the subcutaneous area.
ii. close to the hilum of organs, like the lungs.
iii. along the sides of the trachea and esophagus.
iv. in the axillary, inguinal, and retroperitoneal regions.
v. on the sides of the aorta and inferior vena cava.
vi. They are usually close to the blood vessels because the arteries, veins, and lymphoid vessels run together.
vii. The main function of the lymph nodes is to filter the lymph.
viii. Lymph vessels start blind in the tissue spaces. Lymph nodes are in the way of the lymphoid vessels and so the lymph is filtered through these lymph nodes.
c. The spleen is located on the left side under the diaphragm. Its main function is to filter the blood.
d. The third secondary lymphoid tissue is the lymphoid follicles and larger conglomerates of lymphoid follicles that are associated with specific organ systems. These are the Mucosal Associated Lymphoid Tissue (MALT) and the GI tract Associated Lymphoid Tissue (GALT). These are the parts of the secondary lymphoid system that provide the first defense against external antigens that reach our system either through alimentary Ags or through the air if they are present in the respiratory system.
V. Organization of secondary lymphoid tissues [S5]:
a. The secondary lymphoid organs have special organization.
b. If it is just a diffuse collection of lymphocytes, macrophages, and other accessory cells in different organs and the lamina propria, then you usually will see dark, small lymphocytes in a small concentrated area.
c. Lymphoid follicles are aggregates of lymphoid cells which contain both B and T cells and APCs. It has all the cells of the immune system. Lymphoid follicles are surrounded by lymphoid capillaries because lymphoid capillaries are the first system that the lymph flows through.
d. Lymph nodes are more highly organized secondary lymphoid organs. They have a stroma, which is composed of reticular cells. In the openings and…. (inaudible word at min 5:21) ….will lie the lymphocytes, macrophages, and APCs.
e. The spleen has a capsule. It is a separate organ; it is not in close association with other organ systems.
VI. Lymphatic system [S6]:
a. The lymphatic system starts blindly in the tissue spaces.
b. From the tissue space, the lymphatic capillaries collect Ags that come through the alimentary tract or the skin to reach our system.
c. The lymphoid capillaries will carry the Ag to the lymphoid follicles or to the lymph nodes, which have hundreds of lymphoid follicles covered by a CT capsule.
VII. Lymph Nodes [S7]:
a. This is a cartoon of a cross-section of the lymph node.
b. Lymph nodes have a CT capsule. The CT capsule will send its trabeculae deep into the lymph node. The trabeculae are connected to the reticular cells, which compose the stroma of the lymph node.
c. What is important is that the convex part of the lymph node is where the afferent lymphatic vessels (which carry lymphocytes and Ags) will reach the lymph node by piercing the capsule and opening into the marginal sinuses.
d. The marginal sinuses are where the Ags and the lymphoid cells will reach the lymph node. From there, the cells and the Ags will move down from the trabecular sinuses because the CT of the trabeculae moves down and the sinuses follow.
e. From the sinuses, the cells can migrate out into the lymph node where we will distinguish between B cell zones and T cell zones. The end of the lymph node where the efferent lymphatic vessel leaves the lymph node is called the hilum.
f. Very important to understand that in the immune system in real life, the cells in lymph nodes are constantly moving and bumping into each other. This is how the T cells meet with the APCs so they can recognize the Ag presented specifically for that particular T cell. This is how the B cells meet the Ag that is carried in the lymph. This is how the memory cells can go from one part of the body to another. If the Ag meets that particular memory cell in a different region, it can recognize it. These cells (lymphocytes, APCs, macrophages) are constantly moving between the different lymphoid organs of the system.
VIII. Lymph node [S8]:
a. This is a low magnification cross-section of the lymph node.
b. Underneath the capsule, you see the more open area, this is the marginal sinus. It reaches down following a trabecula of CT. She pointed out the trabecular and medullary sinuses.
c. The darker areas are the medullary trabeculae of the lymph node.
d. Right under the marginal sinus or “subcapsular sinus,” the area with the conglomeration of dark, small lymphocytes with the light center is the B cell zone of the lymph node. This area represents the secondary lymphoid follicle.
e. Each lymph node contains hundreds of lymphoid follicles. A lymphoid follicle is the B cell area of the lymph node. If the center of the lymphoid follicle is lighter, then the cells are proliferating and the lymphoid follicle is active. The center that is lighter is called the germinal center.
f. This picture shows the different areas of the lymph node. There is no real border between the B and T cell zones. There are lots of APCs in between them that can move and find the antigen they are specific for.
IX. Secondary Lymphoid Follicle [S9]:
a. This is a higher magnification of the secondary lymphoid follicle. Remember that a lymphoid follicle is part of a lymph node, and there are many of them in one lymph node.
b. The germinal center is where the cells are proliferating. Once the B cells are activated, they start to divide and grow larger. The nucleus also grows larger because the chromatin is more diffuse. This is why the germinal center area is lighter in color—because the cells are larger and the nuclei are larger and lighter.
c. Germinal center formation occurs when the cell that was specific for a certain Ag met that certain Ag and started to proliferate. Each center is responsible for the development of one specific B cell that will later develop into the plasma cell that will secrete antibodies. It is believed that in each center, only one Ag specific B cell will proliferate, so each germinal center is responsible for the development of one specific antibody.
d. This darker area is the mantle zone. These are small lymphocytes that have not met their specific Ag yet. If this particular lymphocyte meets the Ag, it will start to form a germinal center around it.
X. HEV [S10]:
a. Around the follicles is the T cell zone. In the T cell zone of any secondary lymphoid organs, we see this very interesting structure called high endothelial veins (HEV).
b. Lymph nodes are vascularized. Although the lymph nodes are responsible for filtering the lymph, the cells are able to move between the lymphoid vessels and the blood vessels in the lymph nodes. These HEV are the sites of the movement of cells between the lymphoid system and the blood stream.
c. The cells can move both directions: they can come out of blood into the lymph node, meet the Ag, and get activated or cells from the lymph node can get into the blood stream and the blood can carry those cells that have been activated into the tissue spaces where the cells get filtered out and start to proliferate there to provide the systemic response.
d. HEVs are the sites where the lymphoid system and the circulatory system meet in the lymph nodes. They are more numerous around the follicles in the T cell areas of the lymph nodes.
e. They are endothelial cells that carry endothelial cell markers, but they don’t look like endothelial cells because they are taller. Endothelial cells of capillaries would be flat, and you would not see the cell so well. The difference here is that they are higher, they look like cuboidal cells, but they are blood vessels, they are venules.
XI. Reticular cells and macrophages [S11]:
a. This is one of the sinuses (marginal, subcapsular or trabecular). What is important is that the sinuses are spanned by reticular fibers. Reticular fibers are produced by reticulocytes, and these fibers span through the wall or the capsule of the lymph node.
b. The inner area is where there are more lymphocytes, and the fibers form a network in which the cells lie.
c. In the sinuses, you see a bunch of macrophages and debris. You cannot distinguish between macrophages and APCs in this slide because you can only tell the difference when the cells are stained for MHC II, which only presents on the surface of the APCs. If this phagocyte is MHC II+ which binds to CD4, then these are APCs. They can migrate into the stroma of the lymph node, stay there, and present the Ag. If they are just macrophages that function solely to chew up the Ag, then they just move around with the lymph to other tissues and phagocytose and destroy Ags.
d. Silver staining of the tissue shows the reticular fibers, which stain black.
XII. Lymph Nodes (B cell activation) [S12]:
a. Lymph nodes filter the lymph. The Ag enters the lymph node through the afferent lymphatic vessels. Then it reaches down into the T and B cell zone where the Ag meets the cells, and it either meets the right cell or not. If they do meet, that particular cell starts to proliferate.
i. If it is a B cell, it will develop into a plasma cell. The plasma cells will move down into the trabeculae in the medullary region of the lymph node. Here (pointing at diagram) you see the B and T cell zone, this is the cortex. There you see the trabeculae and lots of sinuses; this area is the medullary region of the lymph node. The plasma cell will secrete the antibodies into the sinuses in the medulla.
b. The B and T cells are able to leave the lymph node through the efferent lymphatic vessel.
c. Remember that the blood stream and the lymphatic circulation are connected in the lymph node in the T cell area at the specialized structure called the HEV. Through the wall of the HEV, the T and B cells and macrophages can migrate in and out, and that is how the connection between the blood stream and lymphatic vessel happens.
XIII. Spleen [S13]:
a. The spleen is a secondary lymphoid organ that filters blood. The spleen is located on the left side underneath the diaphragm.
XIV. Spleen [S14]:
a. It looks like a large bean-shaped organ that has a hilum. This is where the splenic artery reaches the spleen and where the splenic vein leaves the spleen. The spleen is covered by CT capsule. The Ags will reach the spleen through the lymphatic artery and the splenic artery.
XV. Spleen #2 [S15]:
a. This is a cartoon of a cross-section of the spleen. It is covered by a CT capsule that sends these trabeculae (pointing at the diagram) down deep into the organ. After it reaches the hilum of the spleen, the artery divides into smaller branches. The spleen arteries will run in the center of the lymphatic tissue.
b. The area that surrounds the splenic arteries is the T cell zone (meaning that it contains mostly T cells); this area is also called the periarterial lymphatic system or sheath (PALS).
c. Outside of the T cell zone we will find B cell zones or lymphatic follicles, depending on whether it is active or inactive. It is a conglomerate of B cells, and if one of them gets activated, it starts to proliferate and form the germinal center. If they are not activated, it is just small dark lymphocytes; it is darker than the surrounding area.
d. Outside of the B cell zones, we see the marginal zone where there are lots of sinusoids. Sinusoids are where the cells are moving in and out in between the vessels.
e. The white pulp of the spleen is the lymphatic system of the spleen. The white pulp (which can be divided into the T cell zone (PALS), lymphatic follicles, and marginal zone) is located right around the arteries that divide in the spleen.