What are the sites of formation of blood cells in the fetus?
- Yolk sac from week 4 of development.
- Liver until shortly before birth.
- Spleen until cartilagenous bones vascularised.
Where in the adult/infant life are blood cells produced?
- marrow of most bones in children.
- mainly marrow of pelvis, sternum, vertebrae, and cranial bones in adults (due to fat deposition in marrow of long bones).
Where in bone marrow would you find magakaryocytes?
What are primary lymphoid tissues?
They are sites where lymphocytes differentiate to express antigen receptors:
- Thymus (T lymphocytes).
- Bone Marrow (B lymphocytes).
- Describe the structure of the thymus
The thymus is a specialised primarily lymphoid organ of the immune system.
Within the thymus, T cells mature.
T cells are critical to the adaptive immune system.
In the medulla, the network of reticular cells is coarser than in the cortex, the lymphoid cells are relatively fewer in number, and there are concentric, nest-like bodies called Hassall's corpuscles.
These are concentric, layered whorls of epithelial cells that increase in number throughout life. They are the remains of the epithelial tubes, which grow out from the third pharyngeal pouches of the embryo to form the thymus. In the center of the medullary portion there are very few vessels, and they are of minute size.
The medulla is the location of the latter events in thymocyte development. Thymocytes that reach the medulla have already successfully undergone T-cell receptor gene rearrangement and positive selection, and have been exposed to a limited degree of negative selection.
The medulla is specialized to allow thymocytes to undergo additional rounds of negative selection to remove auto-reactive T cells from the mature repertoire.
Transcriptional regulators AIRE and FEZ2 are expressed by the thymic medullary epithelium, and drives the transcription of organ-specific genes such as insulin to allow maturing thymocytes to be exposed to a more complex set of self-antigens than is present in the cortex.
What are secondary lymphoid tissues?
Secondary lymphoid tissues are specialised sites for turning on the acquired immune response:
- Lymph Nodes
- Mucosal Associated Lymphoid Tissue (MALT).
- Gut Associated Lymphoid Tissue (GALT).
- Nasal Associated Lymphoid Tissue (NALT).
What is the lymphatic system?
A drainage system involved in fluid balance, returning to the blood
- excess interstitital fluid
- plasma proteins.
- Originate as 'closed tubes' in almost all tissues (except CNS, epidermis and cartilage).
- Capillary wall constructed of overlapping endothelial cells that respond to fluid pressure.
Lymphatic system works solely on local pressure to drive fluid into local vessels.
How are lymphatic vessels articulated?
- Form joining lymph capillaries.
- Superficial lymphatics follow superficial veins
- Flow into lymph nodes in axillary (armpit) inguinal (groun) or cervical (neck) areas, where they drain into deep lymphatics.
- Deep lymphatics follow main vessels
- lymph nodes either side of the aorta (para-aortic) drain the paired organs, nodes lying anterior (pre-aortic) the gut etc.
Identify the lymph nodes involved in superficial lymphatic drainage.
- The trunk can be divided into 4 quadrants with different drainage by vertical and horizontal lines through the umbilicus.
Identify areas of deep lymphatic drainage.
Lymph from three quarters of the body drains into the left brachiocephalic vein via the thoracic duct.
Lymph from the upper right quadrant enters the right brachiocephalic vein.
What is immunological surveillance?
The same lymphocyte can be in armpit one minute, toe next minute. But how do they achieve this surveillance?
Through having a dual circulatory system.
Lymphocyte arrives at lymph node, passes through lymphatic vessels.
Lymphocytes don't have to be everywhere, they can circulate. But lymphatics drain absolutely everywhere, and those lymphatics drain into the lymph nodes. So lymphocyte dosen't need to go into toe to know that you have e-coli infection in your toe - as long as it goes to lymph nodes which is supplied by lymphatics that drain into your toe, then you can get information from the lymphatic fluid about any insults. Its quite an efficient system.
What are the vessels that go in and out of lymph nodes?
Afferent vessels - go into lymph nodes.
Efferent vessels - go out of lymph nodes.
What do lymph nodes do?
- An important component of host defense.
- Filter lymphatics
- LN represents the anatomical meeting place for cells of immune system and their exposure to antigen.
Describe the structure of a lymph node (real histology)
Describe the structure of gut-associated lymphoid tissue.
Describe the cellular traffic present in a lymph node when draining an infection.
What is the spleen?
The spleen is the largest lymphoid organ
- Directs immune rsponses to antigens in the blood.
- Important for clearance of effete red blood cells.
- Hyposplenism or asplenia not fatal, but predisposes to infection (especially of capsulated bacteria).
- Presence of howell-jolly bodies (erythrocytes with nuclear fragments) in peripheral blood is often diagnostic of hyposplenism/asplenia.
- Splenomegaly common in chronic disease (malaria, leishmaniasiss, Hodkin's disease).
Describe the anatomy of the spleen.
- Left hypochondriac region of the abdominal cavity
- normally around 12 x 7 x 2.5cm and 0.2kg
- 'accessory spleen' may be present in 10% of the population
Identify the afferent splenic artery
What are the functional similarities between the spleen and the lymph node?
What are 'stromal cells'?
They are non-haematopoietic cells which form the underlying structure of lymphoid tissue and and direct the behaviour of hematopoietic cells in lymphoid tissue.