The lymphatic system - Lecture notes 9 PDF

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Summary

an overview of the lymphatic system ...

Description

The lymphatic system The lymphatic system A network of vessels and tissues that assist in circulating fluids and the immune response - Closely associated with the cardiovascular system , fluid that moves out of the blood vessels into the tissues ( lymph ) is transported back into the blood stream - it helps defend the body against diseases , tissue fluid passes through the lymph nodes before it returns to the blood stream so that any pathogens can be recognised and in immune response triggered - Helps in transport of fats and fat soluble vitamins - not present in the CNS or bone marrow

Lymph Movement - Hydrostatic pressure of tissue fluid drives the lymph into the lymphatic capillaries - muscle activity largely influences the movement of lymph through the lymphatic vessels via: - action of skeletal muscles , respiratory movements , smooth muscle in the large lymphatic vessels - valves in lymphatic vessels prevent backflow Lymphatic vessels - The walls are similar but thinner than those of veins - lymphatic vessels are composed of 3 layers an endothelial lining ( inner , smooth muscle ( middle ) , connective tissue ( outer ) - larger vessels lead to lymph nodes and then to larger lymphatic trunks

Mostly clear fluid except near the gut where special lymphatic vessels (LACTEALS) allow fat absorption. This cloudy lymph is called CHYLE

The lymphatic system

SMC- smooth muscle cell BM- basement membrane LEC – lymphatic endothelial

Lymphatic Trunks and Collecting Ducts \ The trunks drain lymph from the lymphatic vessels \ - Named for the regions they serve e.g. lumbar , intestinal , intercostal , Broncho mediastinal , subclavian , and jugular

The lymphatic system

Lymphatic capillary > Afferent lymphatic vessel > lymph node > efferent lymphatic vessel > lymphatic trunk > collecting duct > subclavian vein Lymph formation - Filtration from the plasma normally slightly exceeds reabsorption , leading to the net formation of tissue fluid - increased tissue fluid hydrostatic pressure within interstital spaces forcing fluid into lymphatic capillaries - this process prevents accumulation of excess tissue fluid or oedema - caused by inflammation or blockage In certain circumstances odema can occur in the absence of inflammation e.g. pulmonary odema due to left ventricular cardiac failure , obstruction to lymphatic drainage - elephantiasis due to parasitic worm infection e.g. filairisis - blisters can form when the skin has been damaged by friction , heat , cold or chemical exposure Odema also occurs during the inflammatory response to infection or injury - During inflammation a number of vasoactive mediators are produced which increase the permeability of the post capillary venule  Tissue and fluid lymph Once tissue fluid has entered a lymphatic capillary it is termed Lymph  Lymph formation depends on tissue fluid formation  Capillary blood pressure filters water and small molecules from the plasma 

The resulting fluid has:

The lymphatic system  Much the same consistency as plasma  Contains water and dissolved substances, with the exception of some plasma proteins  The exception is chyle in the lacteals Lymph nodes

Lymphoid organs - Primary lymphoid organs are tissue in which lymphocytes develop and mature - Bone marrow ( B cells ) - Thymus ( T cells ) Secondary lymphoid organs filter potentially harmful particles from the lymph and provide the environment for lymphocytes to be activated - lymph nodes are secondary to lymphoid organs - Other secondary lymphoid organs include the spleen and varioud mucosal associated lymphoid tissues ( MALTs) e.g. tonsils The Spleen - the largest lymphatic organ - is located in the upper left abdominal quadrant

The lymphatic system - has sinuses filled with blood contains 2 tissue types , white pulp ( lymphocytes ) , red pulp ( red blood cells , lymphocytes and macrophages ) - filters blood not lymph

Red pulp - mainly macrophages , red blood cells with few lymphocytes ( phagocytosis of old / dmaged RBC’s ) White pulp - periarteriolar lymphoid sheath ( PALS ) surrounds splenic arteries ( predominantly T-cells , important in initating response ) - marginal zone : edge of the PALS made up of mainy B- cells , contains 1 degree lymphoid follicles

Primary lymphoid organs Bone marrow is the site of haematopoiesis B cells mature here The Thymus is the site of T cell maturation

Thymus The thymus is

The lymphatic system - larger in infancy and during puberty - small in an adult - replaced by fat and connective tissue in the elderly - secretes protein hormones called thymosin’s , which stimulate maturation of T lymphocytes

Defences against infection The body can prevent entry of pathogens or destroy them with defense mechanisms such as : Innate defences Barriers , innate cells and proteins that act immediately to prevent pathogen entry and growth The same response on first and any subsequent infection Adaptive defences T cells and B cells On first infection takes around 5-7 days to be effective On subsequent infection response is very fast Barriers Mechanical - The skin and mucous membranes create mechanical barriers - ciliated mucosa keep mucosa moving out of the body Chemical Ph: E.G. slightly acidic skin ( acid mantle ) very acidic stomach acid Enzymes: specific for different barriers break down pathogen components Mucous : sticky , traps pathogens may also contain enzymes , bacteriostatic agents Innate Immunity Cells : Phagocytes bind and internalise pathogens before digesting them - macrophages , neutrophils and dendritic cells NK cells can recognise ‘ stressed cells ‘ and kill them , including virally infected cells - Granulocytes contain defensive chemicals

The lymphatic system and can be triggered by other parts of the immune system , important in inflammation and allergy Non-cellular : Complement is a cascade of proteins that can punch holes in pathogen membranes Can help other immune cells to ‘ see’ pathogens Helps to cause inflammation Fever Activated innate cells produce chemical mediators which have other effects on the immune system E.g. cytokines interleukin1 ( IL-1) and tumour necrosis factor alpha ( TNF-a) Many effects including - increase metabolism to produce heat - Act on the hypothalamus to allow body temperature increase ( fever ) - Allows immune cells to be more effective - Pathogen growth will be slower - If too high can be dangerous Adaptive immunity - Slower responses that innate response ( 5-7 days ) - Can fight almost all pathogens - Response is specific against the invader , targeted against pathogen molecules ‘ antigens ‘ - B CELLS PRODUCE ANTIBODY - T cells can produce cytokines to direct the immune response or directly kill infected cells - Immunological memory - Some B and T cells stay in the body so if the same pathogen infects again the response is faster B cells and antibody B cells are produced in the bone marrow then circulate the body, most go to lymph nodes - Activated in the lymph nodes - Produce antibodies which bind pathogens and make them more visible to other immune cells - antibodies are specialised depending on type and location of pathogen Tcells -Produced in the bone marrow , mature in the thymus then circulate the body , most go to lymph nodes - Activated in the lymph nodes - Killer T cells ( cytoxix lymphocytes ( CTLs) , CD8+ T cells ) - Cause infected cells to undergo apoptosis ( ‘ commit suicide ‘ ) Helper T cells ( CD4+T cells ) - Produce chemical messengers for the rest of the immune system Activating the adaptive immune system Antigens can travel through the lymphatic system or the blood to secondary lymphoid tissue - Taken up and presented by phagocytes to T and B Dendritic cells at the site of infection take up antigen - Migrate to the lymphatic tissue and present to T and B cells Those T and B cells that recognise antigen with proliferate and become activated

The lymphatic system

The immune response and immunological memory First adaptive immune response is slow 5-7 days - Due to need for T cells and B cells to be activated and proliferate - Second adaptive immune response is faster - Some activated T and B cells remain in the system - React faster - More specific

Inflammation - The inflammatory response consists of :

The lymphatic system A vasodilation – more blood per area of tissue , flows more slowly B Increased vascular permeability – causing vessels to become leaky C Leakage of plasma from blood to tissues D Emigration of inflammatory cells ( mostly neutrophils ) from blood to tissues

The lymphatic system Summary Lymphatic vessels play a role in : -Absorption of dietary fats -Delivering fats to the bloodstream -Collecting of excess interstitial fluids - Delivering excess fluids to the bloodstream - Delivering foreign particles to the lymph nodes where B cells and T cells are activated Primary lymphatic tissue produces B cells and T cells Secondary lymphatic tissue e.g. lymph nodes allow B cells and T cells to become activated When tissue is damaged or infected chemical mediators are released which trigger inflammation - The areas becomes red , warm and tissue fluid increases - Helps to flush antigen to the lymphatics - Inflammatory cells enter the tissue - First innate cells ( neutrophils then monocytes/macrophages ) - later T cells In the lymph nodes B and T cells are activated and become functional after approximately a week On subsequent infection of the same pathogen B and T cells responses are much faster due to memory...