Lymphoreticular system

Lymphoreticular system notes for Veterinary Medicine

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Functions of the Spleen

The spleen is the largest of the lymphoid organs. It is a secondary lymphoid organ as it is the site of lymphocyte activation

Functions:

  • Storage of blood
  • Destruction of old red blood cells
  • Red pulp - removal of particles from circulation and it mostly filled with blood
  • White pulp - sits of activation of lymphocytes and mostly made of white blood cells which screen the blood flowing through using b and t lymphocytes
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Features of the Spleen

Features:

  • Smooth muscle capsule surrounding it so that it can contract and push blood out into the circulation
  • Trabeculae which provide internal support for the spleen and carry blood vessels into the spleen
  • Central arteries for blood supply
  • White pulp contains lymphoid aggregations, mostly lymphocytes and macrophages which are arranged around the arteries.
  • Red pulp is vascular, and has parenchyma and lots of vascular sinuses which are sinusoids - a specialised type of capillary which is very leaky
  • The lining endothelial cells have wide slits between their lateral margins, that act as a filter. The blood cells have to move through these slits, before they can leave the spleen and worn out/defective blood cells are damaged syring the process. The damaged cells are then phagocytosed by the macrophages in the red pulp, that lie just next to the sinusoids
  • Germinal centres are sites wihtin secondary lymphoid organs - lymph nodes and the spleen - where mature B cells proliferate, differentiate, and mutate their antibody genes, through somatic hypermutation aimed at achieveing higher affinity) during a normal immune response to an infection
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Thymus

Primary lymphoid organ located in caudal neck and cranial thoracic regions.

It is larger in younger animals to kick-start development of the immune system. It does regress with age and is later replaced with fat tissue (this makes the thymus difficult to see in older individuals).

There is a variation in appearance between species.

Features

  • Cortex, medulla
  • Connective tissue capsule form incomplete lobules
  • Hassall's corpuscle - eosinophilic type VI epithelial reticular cells arranged concentrically
  • Bone marrow derived lymphocytes travel into the cortex of thymus
  • Here they mature into T cells
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Primary and secondary lymphoid tissues

Primary lymphoid tissues - Where lymphocytes are produced and mature into

  • B cells - bone marrow, bursa of fabricius, peyer's patches
  • T cells - thymus 

Secondary lymphatic tissues - Where lymphocytes are activated. Arranged as a series of filters monitoring the contents of the extracellular fluids and also where the lymphocytes are activiated.

  • Spleen, lymph nodes, mucosa associated tissue (MALT), Peyers patches
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Lymph nodes

Located at various points along lymph vessels, act as elaborate filters. Cattle and dogs have fewer, larger nodes whilst pigs and horses have lots of smaller nodes

Structure:

  • Enclosed by a fibrous capsule
  • Possesses capsular extensions called fibrous trabeculae which provide support for blood vessels entering the nodes
  • Germinal centers are where antigen activated B cells diversify their immunoglobin genes by somatic hypermutation to generate high-affinity antibodies. They proliferate into into antibody secreting plasma cells
  • Primary follicles: Lymphoid follicles without a germinal centre
  • Secondary follicles: Lymphoid follicles with a germinal centre. These mostly contain B-cells.
  • Lymph, containing micro-organisms, soluble antigens, antigen presenting cells and a few B cells, enters the lymph node via afferent lymphatic vessels which enter the subcapsular sinus
  • It then runs through cortical sinuses into medullary sinuses and leaves through the efferent lymphatic vessels at the Hilium as efferent lymph
  • As lymph passes through the node, lymphocytes are added, and macrophages act as filters to remove microorganisms halt the spread of infection and tumours
  • The outer cortex has lymphatic nodules that contain mostly B-cells
  • The inner cortex contains mostly T cells
  • The deep cortical, and medullary cords contain B-cells and plasma cells
  • B cells are mainly found in the outer cortex where they are clustered together as follicular B cells in lymphoid follicles and T cells and dendritic cells are mainly found in the paracortex
  • High endothelial venules - characterised by plump endothelial cells and allow lymphocytes from blood to enter directly into a lymph node by crossing the HEV
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Mucosa associated lymphoid tissue

The mucosa of the digestive, respiratory and urinary tracts often contains small aggregations og lymphoctes called lymphoid follicles. These are the MALT

In some cases the aggregations are large and confluent. This happens in the tonsils, peyers patches and the appendix

It initialtes immune response to specific antigens encountered along all mucosal surfaces. MALT inductive sites are secondary immune tissues where antigen sampling occurs and immune responses are initiated

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Peyer's patches

  • Peyer patches are round or oval and are located in the mucous membrane lining of the intestine
  • Peyers patches mostly contain T-cells, but also can have germinal centres that contain B-lymphocytes, as well as macrophages.
  • They are small masses of lymphatic tissue found throughout the ileum region of the small intestine. They form an important part of the immune system by monitoring intestinal bacteria populations and preventing the growth of pathogenic bacteria in the intestines
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Tonsils

  • Large non-encapsulated masses of lymphoid tissue lying in the walls of the pharynx and nasopharynx, and at the base of the tingue
  • The luminal surface of the tonsils are covered with a stratified squamous epithelium (in common with the oral epithelia)
  • The tonsils have many invaginations which form blind crypts
  • Below the epithelium, there are many lymphoid folicles beneath which have germinal centres like the lymph nodes
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What is the lymphatic system?

The lymphatic system is part of the circulatory system made up of capillaries, vessels, ducts, nodes and tissues. It carries excess tissue fluid back to the circulating blood.

Why do we need the system?

It has three functions: it is a mechanism for removing excess interstitial fluid, transportation of fats and immune defence

Structure of the lymphatic system:

  • Lymphatic capillaries
  • Lymphatic vessels
  • Lymphatic ducts
  • Lymph nodes
  • Lymphatic organs
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Removal of excess interstitial fluid

In capillary beds:

  • Fluid and dissolved gases are passing out of the circulating capillaries into tissues
  • Overall net filtration of fluid - excess is removed from the tissue by the lymphatic system
  • Interstitial fluids enter lymph capillaries via openings between adjacent endothelial cells

Lymph capillaries - lumph vessels (with lymph nodes) - lymph trunks and ducts - circulatory system

Lymph:

  • Fluid from the blood moves into spaces surrounding cells of the tissue - interstitial fluid
  • Similar to plasma
  • Contains more lymphocytes than are present in blood (higher concentration)
  • Lymph also contains pathogens, cell products (such as hormones) and cell debris
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Lymph vessels

  • Flow is unidirectional 
  • Lymph vessels are similar in structure to veins but are more delicate (thinner walls)
  • Valves (many) prevent backflow
  • Flow is mainly passive and is maintained by valves and contraction of surrounding skeletal muscles and arteries
  • Unlikely to see on speciment as the walls are very thin and fluid is clear

Trunks and ducts

  • The thoracic duct is the largest duct of the lymphatic system
  • Originates from the cisterna chyli in dorsal abdomen
  • Chyle is the lymph from the guts
  • Drains into cranial vena cava around vicinity of left brachiocephalic vein
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Swollen lymph nodes

  • Lymph nodes drain specific areas therefore if infection occurs within a drainage area the lymph node will become active 
  • Rapid cell turnover and production of local inflammatory mediators may cause the node to enlarge and become tender
  • Because lymph nodes are efficient filters and flow through them is slow, cells that have migrated from  primary tumours and lymphatic vessels can often lodge and grow as secondary tumours in lymph nodes - metastasis
  • N.B. cancerous cells can metastasise to any region, esp. well vascularised and where blood flow is slower e.g. lungs, liver etc.
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Lymph nodes of the dog

Parotid - with parotid salivary gland. Drains dorsal structures of the head. not normally palpable

Mandibular centre - found in intermandicular space or andle of the jaw. Drains the muzzle, salivary glands and tongue. Drains into the tracheal duct.

Superficial cervical/prescapular - just cranial to the shoulder. Drains wide superficial teritory including proximal forelimb. Drains into tracheal duct

Axillary centre - in axilla, drains deep structures of whole limb and superficial structures of distal limb

Superficial inguinal  - groin and caudal mammary glands

Popliteal centre - popliteal fosa caudal to stifle. Drains distal part of the limb. Draind into centres within the pelvic region

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Lymph nodes of the horse

Retropharyngeal - medial group lie at roof of pharynx. Lateral related to guttural pouch in horse. Drain larynx and pharynx. Collecting centre for other nodes of the head.

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Lymph nodes of the cow

Superficial inguinal (mammary) is important in cows

Subiliac normally large - drains the flank and skin over the thigh and stifle

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MALT and GALT

Secondary lymphoid tissues

  • Peyer's patches - lymphoid tissue specifically located in the wall of the intestine
  • Tonsils - features are crypts and no capsule
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Lymphoid organs in birds

  • Birds have fewer lymph nodes and many species of bird have none
  • Lymphatic vessels also less numerous
  • Thymus has several separate lobes that accompany the jugular veins

Bursa of fabricius (cloacal bursa)

  • Sac liek structure on dorsal surface of cloaca
  • Primary lymphoid organ similarly to thymus regresses with age
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