Lymphatic system Flashcards
Lymph
Lymph: Tissue fluid arising from filtration of blood through the blood capillaries is called lymph once it enters the lymphatic capillaries. It contains small molecules of blood plasma such as water, ions, nutrient molecules and respiratory gases
Lymph is normally a clear colourless fluid, but lymph draining the intestine during absorption is often milky in appearance because of its high lipid content. This is called ‘chyle’.
Lymphatic system
Lymphatic system provides immune surveillance and defence against foreign substances and microorganisms. It helps to maintain fluid balance by accumulating tissue fluid and white blood cells in lymph vessels and returning them to the blood. It drains extracellular fluid from the tissues, returning it to the blood circulatory system after passage through lymph nodes.
Lymph moves sluggishly from the capillary network into the larger lymphatic vessels; back flow is prevented by numerous flap-like valves similar to those in veins.
Components of the lymphatic system
Lymphatic capillaries
lymphatic capillaries- The intercellular spaces of tissues contain small endothelial-lined tubes, which are blind ending. These are permeable to fluids and dissolved molecules in the interstitial fluid. The lymphatic capillary network acts as a drainage system, removing surplus fluid (lymph) from tissue spaces.
Where do the thoracic and right lymphatic ducts empty into?
The larger lymphatic vessels have muscular walls and pump the lymph into the following two main lymphatic ducts.
- Thoracic Duct
- Right Lymphatic Duct
(empties at the junction of internal jugular and subclavian vein)
The cisterna chyli is a dilated sac at the lower end of the thoracic duct in most mammals into which lymph from the intestinal trunk and two lumbar lymphatic trunks flow.
Lymphoid cells
B-cells develop in the bone marrow and are responsible for production of immunoglobulins (antibodies). B cells differentiate into B memory cells and plasma cells, providing humoral immunity.
T-cells originate in bone marrow, mature in the thymus, and subsequently seed secondary lymphoid tissue. T cells differentiate into helper, memory, and cytotoxic cells. T lymphocytes provide cell-mediated immunity and assist B lymphocytes in their humoral response.
NK cells arise from bone marrow and are stimulated by interferon to recognize and kill “non-self” cells (antigens)
Plasma cells differentiate from B lymphocytes and produce humoral antibodies
Dendritic cells and macrophages phagocytose foreign matter, enhance the body’s response to antigen by “presenting” antigen to lymphocytes, and secrete immunomodulatory factors
Lymph node
Each lymph node is a relatively small, soft structure; they have a fibrous connective tissue capsule, usually surrounded by adipose tissue. They are located along the paths of lymph vessels, forming a chain of lymph nodes so that lymph flows from one node to the next
Afferent- located at the hilium, valves that ensures unidirectional flow, carry unfiltered lymph fluid in
Efferent- located at the hilium, lymph exits lymph node here, valves that prevent regurgitation of lymph back into the node, carry filtered lymph fluid out
Medullary cords composed of B lymphocytes that extend from the inner cortex into the medulla
Medullary sinuses allows continuations of the intermediate sinuses in the cortex. Lymph flows from medullary sinuses into the efferent lymph vessels that exit at the hilum of the node.
Lymph node histological diagram
Divided into three regions: cortex, paracortex, and medulla- all have a rich supply of sinusoids, enlarged endothelially lined spaces through which lymph percolates. Lymphoid nodules are characteristic features of lymphoid organs. They occur most typically in the cortex of normal lymph nodes, in the spleen, in the tonsils, and in the Peyer’s patches of the ileum. The subcapsular sinus is the space between the capsule and the cortex, which allows the transportation of the lymphatic fluid .
2 types of lymphoid nodules/follicles (aggregates of B lymphocytes). Primary are in cortex of lymph node (densely stained). Secondary have germinal center in them (less densely stained) that allows generation of B memory cells.
The medulla contains plasma cells and macrophages.
The paracortex is a T cell–rich region that is located between the medulla and the follicle-rich region of the cortex.
A mantle zone or corona is present that represents an outer ring of small lymphocytes that encircles the germinal center. These lymphocytes represent naïve B lymphocytes that have not been previously stimulated by antigen.
Thymus and its components of the outer cortex and inner medulla
Immunologically naive lymphocytes from bone marrow called thymocytes differentiate into mature T cells in the thymus. During this process, the immune system distinguishes self from foreign antigens and develops self-tolerance. Epithelial cells within the thymus secrete protein hormones called thymosins, which stimulate maturation of T lymphocytes after they leave the thymus andmigrate to other lymphatic tissues.
Function and composition of the outer cortex- Maturation of immature T lymphocytes. T lymphocytes gaining immunocompetence. ERCs (epithelial reticular cells) are the primary cells involved with making sure that no T cells are allowed to survive which could attack the body’s own cells.
Function and composition of the inner medulla- Development of self-tolerance by deletion of self-reactive clones of T cells. Lymphocytes (fewer) + Hassal corpuscle (these concentric corpuscles are composed of a central mass, consisting of one or more granular cells, and of a capsule formed of epithelioid cells, composed of type VI epithelial reticular cells). Medulla is lighter b/c cells are further apart
Blood thymus barrier
The blood–thymus barrier consists of three major elements: (1) capillary endothelium and its basal lamina, (2) perivascular connective tissue space occupied by macrophages, and (3) type I epithelioreticular cells with their basal lamina.
These layers provide the necessary protection to the developing immature T cells and separate them from mature immunocompetent lymphocytes circulating in the
bloodstream.
Spleen histology
Tonsil histology
Appendix histology
Red and white pulp
• Red pulp contains mainly macrophages, red blood cells with few lymphocytes. It is a filter, which removes aged and damaged red blood cells and microorganisms from circulating blood. It also is a storage site for red blood cells. It is composed of splenic sinuses and splenic cords (Billroth cords).
– Old and damaged red blood cells are phagocytosed in the red pulp.
• White pulp mainly surrounds the arteries in the spleen, forming a periarteriolar lymphoid sheath (PALS). Immune component of the spleen
– This is predominantly populated by T-cells.
