by
Olukemi A Esan (FMCPath)
Olukemi A Esan (FMCPath)
Types of lymphoid tissues
The primary lymphoid tissues are the Bone marrow and Thymus
Secondary lymphoid tissues include the spleen, lymph nodes and peripheral lymphoid nodules.
The Spleen
Located in the left hypochondrial region
Beneath the 9th-llth ribs.
It weighs between 150-250g in a normal adult but may enlarge in a variety of conditions.
Blood flow to the spleen
Via the splenic artery through it’s branches-trabecular arteries and central arteries which are situated in the white pulp.
These central arteries run in the central axis of the periarteriolar lymphatic sheath derived from the splenic capsule.
The central arteries give off many arterioles and capillaries some of which terminate in the white pulp while others go on to enter the red pulp.
During blood flow in the spleen, plasma skimming occurs
Most of the plasma and leucocytes go to the white pulp
Haemoconcentrated blood goes to the red pulp.
In the red pulp are 2 circulatory systems
A ‘closed’ system where the blood goes from the arteries via the sinuses back into the veins
An ‘open’system where the blood is emptied from the arterioles into the splenic bilroth cords. Here the blood cells pass through a meshwork of reticulum fibres and reticuloendothelial system cells to reach the splenic sinuses by passing through gaps in the endothelial cells lining the sinuses.
The red cells are normally flexible enough to squeeze through the endothelial slits into the sinuses.
Cells with abnormal membranes or inclusions which render them relatively inflexible remain in the cords where they either become conditioned for later transit or are destroyed.
Circulation in the spleen
Lymphatic system
The spleen has no afferent lymphatics
Efferent lymphatic vessels form around the arteries and make their exit in the hilum.
Functions of the spleen
Functions of the spleen include:
Immunologic function
Phagocytosis
Sequestration
Blood pooling
Extramedullary haemopoeisis
Blood vol regulation
Immunologic function
Largest accumilation of lymphoid cells in the body.
25% of the T lymphocytes
15% of the B lymphocytes pool are present here.
T cells are found mainly in the periarteriolar sheaths
B cells are found in the germinal centers of the white pulp.
Destruction of antibody coated rbcs
Phagocytosis & Sequestration
Phagocytosis: This is the uptake of none viable cells and foreign matter by the macrophages.
Sequestration: A reversible process whereby cells are held up temporarily before returning into circulation eg reticulocytes
Blood Pooling
The presence of an increased amount of blood in the spleen which is in continuous exchange with the circulation
eg increased platelet pool in splenomegaly.
Extramedullary Haemopoeisis
Haemopoietic stem cells and progenitor cells are present in the spleen.
They may resume their fetal role as a compensatory erythroblastic hyperplasia eg in chronic haemolytic anaemias and myelofibrosis.
Blood volume regulation
Splenomegaly is associated with an increase in plasma volume which results in a dilutional anaemia.
An increase in the red cell pool in the enlarged spleen may also cause a true anaemia in the peripheral circulation.
Causes of splenomegaly
Haematological
Leukaemias
Lymphomas
Myelofibrosis
Haemolytic anaemias
Others
Acute/chronic infections
Parasitic infections
Collagen dxs
Glycogen storage dxs/lipidosis
Sarcoidosis
Amyloidosis
Portal hypertension
Cysts
Cells with abnormal membranes or inclusions which render them relatively inflexible remain in the cords where they either become conditioned for later transit or are destroyed.
Circulation in the spleen
Lymphatic system
The spleen has no afferent lymphatics
Efferent lymphatic vessels form around the arteries and make their exit in the hilum.
Functions of the spleen
Functions of the spleen include:
Immunologic function
Phagocytosis
Sequestration
Blood pooling
Extramedullary haemopoeisis
Blood vol regulation
Immunologic function
Largest accumilation of lymphoid cells in the body.
25% of the T lymphocytes
15% of the B lymphocytes pool are present here.
T cells are found mainly in the periarteriolar sheaths
B cells are found in the germinal centers of the white pulp.
Destruction of antibody coated rbcs
Phagocytosis & Sequestration
Phagocytosis: This is the uptake of none viable cells and foreign matter by the macrophages.
Sequestration: A reversible process whereby cells are held up temporarily before returning into circulation eg reticulocytes
Blood Pooling
The presence of an increased amount of blood in the spleen which is in continuous exchange with the circulation
eg increased platelet pool in splenomegaly.
Extramedullary Haemopoeisis
Haemopoietic stem cells and progenitor cells are present in the spleen.
They may resume their fetal role as a compensatory erythroblastic hyperplasia eg in chronic haemolytic anaemias and myelofibrosis.
Blood volume regulation
Splenomegaly is associated with an increase in plasma volume which results in a dilutional anaemia.
An increase in the red cell pool in the enlarged spleen may also cause a true anaemia in the peripheral circulation.
Causes of splenomegaly
Haematological
Leukaemias
Lymphomas
Myelofibrosis
Haemolytic anaemias
Others
Acute/chronic infections
Parasitic infections
Collagen dxs
Glycogen storage dxs/lipidosis
Sarcoidosis
Amyloidosis
Portal hypertension
Cysts
Splenectomy
Indications: Rupture, removal of cysts, massive splenomegaly due to Gauchers, Congenital haemolytic states, hypersplenism.
Defer until age 5-6 years if possible
Immunisation against pneumococci, H. influenza and meningococci because of increased risk of overwhelming sepsis and meningitis.
Hypersplenism
Splenomegaly
In one or more cell lines in blood
Normal/hyperplastic BM
Reticulocytosis/large platelets
Increase splenic rbc/plat pool
Confirmed by response to splenectomy
LYMPH NODES
They are encapsulated dense collections of lymphocytes, macrophages, dendritic cells organized along the course of lymphatic vessels.
The marginal sinus is lined by phagocytes.
The cortex contains numerous follicles which are mainly aggregates of B cells.
In the paracortex are the T cells.
The medulla is the site of entry of arteries and veins and contains a mixture of B & T lymphocytes, plasma cells and macrophages.
THYMUS
It developes at about the 8th week of gestation from the 3rd and 4th brachial pouches.
It is located beneath the sternum in the superior mediastinum
Made up of an outer cortex and inner medulla.
Prothymocytes originate in the marrow and migrate to the thymus where they mature into T cells.
Maturation of T cells is accompanied by the sequential acquisition by the thymocytes of the various T cell markers.
A pre T cell is Tdt pos
Gains CD2 , CD4 & 8 in the cortex; gains CD3,rearranges the T cell receptor gene
Losses either CD4 or 8 in the medulla.
A mature T cell is CD 2,3,4/8,Tcr gene rearranged and Tdt negative
The cortex contains numerous follicles which are mainly aggregates of B cells.
In the paracortex are the T cells.
The medulla is the site of entry of arteries and veins and contains a mixture of B & T lymphocytes, plasma cells and macrophages.
THYMUS
It developes at about the 8th week of gestation from the 3rd and 4th brachial pouches.
It is located beneath the sternum in the superior mediastinum
Made up of an outer cortex and inner medulla.
Prothymocytes originate in the marrow and migrate to the thymus where they mature into T cells.
Maturation of T cells is accompanied by the sequential acquisition by the thymocytes of the various T cell markers.
A pre T cell is Tdt pos
Gains CD2 , CD4 & 8 in the cortex; gains CD3,rearranges the T cell receptor gene
Losses either CD4 or 8 in the medulla.
A mature T cell is CD 2,3,4/8,Tcr gene rearranged and Tdt negative
PERIPHERAL LYMPHOID TISSUES
Solitary lymph nodules : no capsule or afferent/efferent lymphatics
mucosa & submucosa of respiratory tract, GIT, Urinary tract and vagina.
The respiratory and GIT mucosal tissues are rich in plasma cells secreting IgA. These nodules store the precursors of the IgA producing cells.
Solitary lymph nodules : no capsule or afferent/efferent lymphatics
mucosa & submucosa of respiratory tract, GIT, Urinary tract and vagina.
The respiratory and GIT mucosal tissues are rich in plasma cells secreting IgA. These nodules store the precursors of the IgA producing cells.
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