Lymphatic System Flashcards
What are the primary or central lymphatic organs?
Bone marrow + thymus
What are the secondary lymphatic organs?
Lymphatic nodules, tonsils, appendix, lymph nodes, and spleen
What are the cells of lymphatic tissue?
B lymphocytes, T lymphocytes, plasma cells, macrophages, and antigen presenting cells
What defines an antigen?
Any substance that can induce an immune response. They can be soluble substances, infectious microorganisms, foreign tissue, transformed or cancerous cells
What happens to antigen-antibody complexes?
They are formed by Ig’s secreted by plasma cells + the respective antigen and are eliminated by the body
What is the innate / nonspecific immune response?
First line of defense: skin, internal epithelial linings, chemical (low pH), secretions like lysozymes and proteins in saliva. If antigens get past these barriers, it is destroyed by other elements of the immune system.
What is humoral vs cellular immunity? What are they a part of?
Both a part of the acquired / adaptive immune response.
Humoral: Secretion of antibodies by plasma cells which differentiate from B cells
Cellular: Activation of T lymphocytes to destroy virus infected cells
What are CD proteins and why are they useful?
They are transmembrane proteins expressed on T cells, called “Cluster of Differentiation”. They can be used to distinguish between T lymphocytes.
CD4 = Helper, CD8 = Cytotoxic
What are MHC molecules? Why do they generally exist for the immune system?
Group of transmembrane proteins which help the immune system recognize self vs non-self
Where is MHC I expressed? What is their function?
On all nucleated cells and on platelets. TH=hey act as a target to allow the elimination of abnormal host cells (viral or cancerous) whenever they present peptide fragments to cytotoxic CD8+ T-lymphocytes.
Where is MHC II expressed? What is their function?
Expressed on surface of antigen presenting cells (APCs) such as dendritic cells and macrophages. They present foreign peptides to CD4+ T-helper cells to activate them.
What is a T-cell receptor (TCR) and how does it work?
They are expressed by T-lymphocytes and recognize a single antigen by interacting with MHC (1 or 2, depending on cell)
What is a B-cell receptor (BCR) and how does it work?
BCRs are antibodies and also recognize a single antigen. After recognizing an antigen, it leads to differentiation of a B lymphocyte to a plasma cell
What does it mean for a lymphocyte to be immunocompetent?
Circulating T and B cells are immunocompetent because they are capable of developing an immune response following exposure to antigen
Where do T-cells differentiate? What happens to them? What is their prevalence?
They differentiate in the thymus by antigen-INDEPENDENT differentiation, where they will become immunocompetent cells. They have a long lifespan and enter the bloodstream / lymphatic system to make up 60-80% of lymphocytes. They can be found in lymphatic tissue or secondary lymphatic organs
What are the functions of CD4+ T Helper cells?
They promote activation and proliferation of B cells, and CD8+ T cells that have encountered their specific antigens
What is the function of CD8+ T cells?
They are cytotoxic, and kill cells infected with intracellular pathogens, or destroy transformed cells or cancerous (tumor) cells
Where do B-cells differentiate? What happens to them? What is their prevalence?
They differentiate in the bone marrow by antigen-INDEPENDENT differentiation, making up 20-40% of lymphocytes. They will move from blood or lymph to become part of lymphatic tissue / secondary lymphatic organs.
What is the mononuclear phagocytic system? What cells are included?
System of cells which recognize foreign molecules, endocytose and process them, then display them on MHCII to T-helper cells.
Includes:
1. Macrophages - CT and lymphatic organs
2. Kupffer cells - liver
3. Langerhan’s cells - epidermis
4. Dendritic cells - spleen + lymph nodes
What are some of the odd functions of macrophages in immune response?
Secretion of cytokines, phagocytosis of bacteria and foreign antigens, can also form a “multinucleate foreign body giant cell” - isolation of certain pathogens via granuloma.
How are lymph vessels formed?
They are formed by networks of blind-ended capillaries in loose connective tissue, especially skin and mucous membranes
What do lymphatic vessels carry? What does it contain?
Lymph, tissue fluid which is a filtrate of blood plasma. Contains:
- Proteins and lymphocytes to return to blood
- Antigens bound to lymph nodes
- Lipids and lipid-soluble vitamins absorbed in GI tract
How can lymph vessels be recognized? How does lymph flow?
They have very thin walls with no RBCs, made of endothelium. It flows by bulk flow (skeletal muscle action and respiratory movement) since there is no pump.
Where does lymph ultimately drain?
After moving into lymph nodes via afferent lymphatic vessels and leaving via efferent lymphatic vessels, it drains into the right or left lymphatic duct which combines with the venous system at the base of the neck. Ultimately it becomes a part of vascular circulation
Where is diffuse lymphatic tissue found? What is it called in the gut?
In the loose connective tissue lamina propria of the GI tract, respiratory tract, and genitourinary tract. It is not enclosed in connective tissue capsule.
GALT = gut-associated lymphatic tissue
What is the function of diffuse lymphatic tissue?
Recognize antigens that have broken through nonspecific barriers.
What are lymph nodules?
Localized concentrations of lymphocytes that can be found in grounds in GI, respiratory, and genitourinary tracts. They are not encapsulated in connective tissue
What is a primary vs secondary lymphatic nodule?
Primary - uniformly distributed and densely basophilic, full of small, inactive B cells.
Secondary - Pale, inner “germinal center” made of large activated B cells. Dark, outer mantle zone / corona which is small B lymphocytes encircling the germinal center.
Where are the permanent locations of secondary lymph nodules?
Outer cortex of lymph nodes, appendix, ileum of small intestin, Peyer’s patches, spleen white pulp, and tonsils.
Though they can form anywhere in the body
What are tonsils?
Aggregates of primary and secondary lymphatic nodules with an epithelial surface covering and a connective tissue base
What is the tonsillar ring?
The ring of lymphatic tissue formed at the entrance of the oropharynx by the summation of the tonsils
What type of epithelium is shared by pharyngeal and tubal tonsils?
Pharyngeal = adenoids
They both share respiratory epithelium
What type of epithelium is shared by palatine and lingual tonsils?
Stratified squamous nonkeratinized epithelium (since it is in the mouth, lining mucosa)
What are tonsillar crypts?
Tonsillar epithelium dips into underlying connective tissue. A band of connective tissue separates tonsils from other tissues
How is lymph drained from tonsils?
They do not posses afferent lymphatic vessels, but lymph is drained by efferent vessels
What are Peyer’s patches? What is similar?
Aggregates of primary + secondary lymph nodules found in ileum of small intestine. Part of several mechanisms present in the intestine to recognize and eliminate antigens
Similar: CT of appendix is also heavily invested with lymphocytes and contains many lymphatic nodules
What are the functions of lymph nodes?
- Filter lymph - all lymph passes through at least one lymph node on its way to the lymphatic ducts
- Remove bacteria and other particulate matter from lymph
- Produce antibodies via plasma cells
- Storage of T and B cells
How does connective tissue invest lymph nodes?
Encapsulated by dense irregular CT making it a secondary lymphatic organ.
Connective tissue extends into the lymph nodes as trabeculae - seen as eosinophilic structures on H/E
What is the function of reticular cells in lymph nodes?
They secrete Type III collagen which stains argyrophilically, provides the reticular connective tissue which organizes and surrounds the lymph node parenchyma
What are the three divisions of the lymph node parenchyma?
- Outer cortex
- Paracortex
- Medulla
What is the outer cortex of the lymph node and what is in it? What is it referred to as?
Parenchyma just deep to the capsule. Contains primary and secondary lymph nodules with B cells, reticular cells, plasma cells, and macrophages. Follicular dendritic cells (APCs) will be in germinal centers (activating B cells).
Many B cells - thus called “B cell dependent region”
What is the deep cortex of the lymph node and what is in it? What is it referred to as?
Parenchyma lying between superficial cortex and medulla. It lacks lymphatic nodules, is a zone of densely packed mainly T cells, but others may be present. Dendritic cells are localized here, as well as high endothelial venules.
Referred to as “T cell dependent region of lymph node”
What is the medulla of lymph nodes and what is in it?
A network of medullary cords (dark) and medullary sinuses (pale)
Cords - plasma cells, macrophages, T cells, dendritic cells, and reticular cells
Sinuses - channels lined with endothelium that drain lymph out of the node
There is a high concentration of antibodies in the medulla because of the plasma cells
How does the endothelial lining of the lymph sinuses differ next to the connective tissue vs parenchyma?
Next to connective tissue: Endothelium is continuous
Next to sinuses: Endothelium is discontinuous
What spans the lumen of the sinuses of lymph nodes and why?
Macrophages and reticular cell processes + by reticular fibers. Causes lymph to slow down as it flows through the sinuses so particulate matter and degenerating cells can be removed
How does lymph flow through a lymph node? Which sections have valves?
- Afferent lymphatic vessels (with valve) pierce capsule and bring lymph into node.
- Subcapsular sinuses move lymph under the capsule
- Trabecular sinuses are located along trabeculae
- Medullary sinuses are between medullary cords
- Efferent lymphatic vessels (with valve) exit lymph nodes at the hilus
What are high endothelial vessels (HEVs) and what is their function?
A type of postcapillary venule in the paracortex of lymph nodes. They are lined with cuboidal / columnar instead of squamous endothelium.
Function: Site of entry (via diapedesis) of T / B lymphocytes from blood into the lymph node. T cells remain in paracortex, B cells migrate to outer cortex.
This causes swelling during infection
How can lymphocytes leave a lymph node?
By entering into the lymphatic sinuses then getting into efferent lymphatic vessel. They CANNOT leave by entering the blood vascular system.
What is the thymus? Where is it?
A bilobed, encapsulated primary lymphatic organ for T lymphocyte differentiation. It is located in the superior mediastinum.
How does organization of the thymus differ from lymph nodes?
Same CT capsule + trabeculae, but it has no reticular cells + fibers. They are organized by epithelioreticular cells held together in a network via desmosomes, and there are 6 different types.
What is the function of epithelioreticular cells of the thymus?
Take the place of the reticular cells of lymph nodes, and function to provide physical support for developing lymphocytes.
However, they also provide metabolic support and induce differentiation of T cells.
What are the two divisions of the thymus? What are their histological components?
Cortex - Strongly basophilic, contains developing T-lymphocytes (thymocytes) + epithelioreticular cells. Organized into lobules via CT trabeculae
Medulla - paler staining, with immunocompetent T cells + epithelioreticular cells. It is continuous through all the lobules that are separated by trabeculae.
What are Hassell’s corpuscles?
Also called thymic corpuscles, they are a unique histological feature of the thymic medulla composed of concentric layers of flattened epithelioreticular cells.
What are the requirements for a developing thymocyte? What percent make it to maturity?
- Express correct CD molecules
- Express correct T cell receptor
- Not recognize self-antigens
98% do not fulfill these requirements and die by apoptosis / are phagocytosed
How to T cells enter circulation from the thymus?
If they have developed correctly, they enter the medulla.
- Enter bloodstream through postcapillary venules
- Enter lymph through efferent lymphatic vessels at corticomedullary junction
What are two mechanisms that the thymus uses to prevent antigens from interacting with developing thymus?
- Thymus lacks afferent lymphatic vessels
- Capillaries in cortex are part of blood-thymus barrier. They are continuous with tight junctions, have endothelial basal lamina + intervening macrophages, as well as a lining epithelioreticular cell layer + its basal lamina.
How does the thymus change throughout life?
Greatest relative weight at birth, and greatest absolute weight at puberty. It begins to curl up and become infiltrated with adipose + connective tissue throughout life.
What is the spleen + its functions?
Largest lymphatic organ, it is an encapsulated secondary lymphatic organ. It is not essential for life.
- Filtration of blood by macrophages + dendritic cells to remove antigens + debris
- Recycling of RBCs
- Antibody production
- Platelet and erythrocyte storage
- Hemopoiesis during fetal development
What is the connective tissue make-up of spleen like?
Very similar to lymph nodes, with capsule, trabeculae, and stroma of reticular cells + fibers
What is a central artery of the spleen and what does it arise from? What does it make?
The splenic artery branches into trabecular arteries then central arteries. These central arteries are arterioles which the white pulp is organized around. It appears eosinophilic, while white pulp appears basophilic
What is PALS?
Periarterial lymphatic sheath - the sleeve of T-lymphocytes around the central artery in the spleen.
-T-cell dependent region of spleen. Part of white pulp
What makes up the spleen white pulp?
It is the area around the central artery (arteriole). Surrounded by periarterial lymphatic sheath (PALS). It also contains primary and secondary lymphatic nodules. These nodules make up the B-cell dependent region of the spleen, since they contain B-cells, macrophages, and APCs.
What is splenic red pulp?
Located between and around white pulp, it appears eosinophilic. It is made up of splenic cords (red pulp cords) and splenic (venous) sinuses
What are splenic cords?
Red pulp, they are a loose meshwork of reticular cells and fibers also containing erythrocytes, macrophages, lymphocytes, other WBCs, dendritic cells, and plasma cells
What are splenic sinuses? What cells does it contain?
They are special sinusoidal vessels. Endothelial cells lining the sinusoids are long and rod-shaped, oriented along longitudinal axis of vessel. However, there are perisinusoidal loops of basal lamina which go around in circumferentially and allow for the easy passage of blood cells in and out.
Macrophages have processes which extend in to monitor passing blood for antigens
What is the difference between open and closed circulation of blood into the spleen?
Open circulation - thought to be the most common mechanism. Blood capillaries open directly into red pulp until reaching a splenic sinus, which allows for most filtration and processing by macrophages.
Closed circulation - Blood is always enclosed in a vessel and thus no exchange with red pulp occurs until splenic sinus, which is limited
What is hemosiderin?
An iron breakdown product of RBCs which stains black and can be found in macrophages of the spleen (red pulp)
