L02: Architecture and Haematopoiesis Flashcards
Lymphatic system composition and purposes (3)
- Lymph vessels: drainage
- Lymph nodes: filtration and defence
- Lymph organs: filtration and defence
Lymph composition (3)
- Intercellular/interstitial fluid
- Proteins
- Particles (subcellular debris, pathogens and cancer cells)
Lymphatic vessels
Drainage function. Lymph transport is slow in vessels and is maintained by skeletal muscle contractions. It is a one-way fluid flow to heart because lymphatic valves prevent back flow.
Lymphatic system
Lymph vessels collect fluid, filtred through lymph nodes, drain into lymph ducts and returned to venous system through thoracic duct to the heart
Lymphedema
Blockage of lymphatic drainage resulting in swollen limbs. Uncontrolled infections may develop. Lymphedema is caused by tissue scarring, surgery (e.g. breast cancer) and parasitic infection (e.g. elephantitis)
Lymphocyte recirculation
Lymphocytes recirculate from blood to the lymph and back again. They recirculate from the bloodstream into the lymph via lymph nodes. Found in both lymphatic and cardiovascular system as they are interconnected. Postcapillary venules in lymphoid organs are composed of cuboidal high endothelial venules HEV) which are sites of lymphocyte extravasation. These cells express a variety of cell adhesion molecules - homing receptors to bind to endothelium.
Naive lymphocytes constantly recirculate, enter lymph nodes from blood. During an infection, Ag is taken up by dendritic cells taken to draining lymph node. In the LN, DCs display Ag to T cells while B cells are activated. Once T and B cells have undergone proliferation and differentiation period, they leave the LN as effector cells via efferent lymphatic vessel which drains into the thoracic duct and into bloodstream.
Lymphoid organs
Lymphoid organs are organised tissues containing large numbers of lymphocytes in a framework of nonlymphoid cells. Divided into central (primary) or peripheral (secondary)
Lymphoid and nonlymphoid cell interaction importance
Interactions important for lymphocyte development, initiation of the adaptive immune response, and lymphocyte sustenance
Primary lymphoid organs
Location of where immune cells develop. Lymphocytes arise from stem cells in the bone marrow and differentiate in the central lymphoid organs (B cells in bone marrow, T cells in thymus)
Thymus
Bi-lobed structure where maturation of T cells occurs, differs from other lymphoid organs because it doesn’t directly fight antigens and reaches maximum size at puberty then atrophies. Made up of several lobules consisting of cortex (outer layer) and medulla (inner layer)
Cortex (outer layer)
Immature thymocytes closely associated with branched cortical epithelial cells and scattered macrophages. Immature proliferating thymocytes on outer cortex parts and immature T cells undergoing thymic selection on inner cortex areas
Medulla (inner layer)
Mature thymocytes, medullary epithelial cells, macrophages and dendritic cells. In the medulla Hassall’s corpuscles are sites of cell destruction
Peripheral lymphoid organs
Sites of lymphocyte activation. Lymph carries the antigen to the lymph nodes and recirculating lymphocytes from lymph nodes back into the blood.
Lymph Node
Functions as a filter and active in immune response. Composed of outer fibrous capsule, cortex (B & T cells) and medulla (macrophages and B cells). Located at the points of convergence of the lymphatics system which collects extracellular fluid from the tissues and returns it to the blood.
Lymph Node - Cortex
Contains lymphoid follicles containing B cells. During an immune response, these contain central areas of intense B cell proliferation and are called germinal centres
Lymph Node - Paracortex
Area of diffuse T cells
Lymph Node - Medulla
Medullary cords - macrophages and plasma cells
Afferent lymphatics
Drain fluid from the tissues, carry Ags and phagocytic cells to LNs where they are trapped
Efferent lymphataics
Exit of lymph and lymphocytes
Spleen
Fist-sized organ behind stomach, collects Ag from blood and senescent RBCs. Red pulp is the site of RBC disposal and white pulp is site of lymphoid areas.
Periarteriolar lymphoid sheath (PALS): mainly T cells
Germinal centre: B cells plus dendritic cells/macrophages
Gut-associated lymphoid tissues (GALT)
Comprised of tonsils, adenoids, appendix, peyer’s patches. Functions to collect Ag from the epithelial surfaces of the GIT. Highly dynamic structures: B cell follicles expand as germinal centres are formed, swelling the glands.
Peyer’s Patches
Most important and organised GALT. Ag is collected by sepcialised epithelial cells - multifenestrated or M cells. Most of the tissue is B cells organised into a large, highly active domed follicle. T cells occupy areas between the follicles. Ag enters across a specialised epithelium of M cells.
Lympho-haematopoietic system
In the embryo, Haematopoietic cells are generated at specific anatomical sites, first identified stem cells arise in blood islands of the yolk sac which migrate to foetal liver. At birth, cells migrate to colonise developing bones and establish haematopoietic lineages in bone marrow, and thymus.
Stem Cells
All mature blood cells are derived from stem cells. Have the ability to self-renew. Majority are mature post-mitotic end cells and so in addition to proliferation, extensive amplification of various compartments must occur. Achieved via generation and subsequent proliferation of committed Progenitor Cells.
