2- Immune Cells And Organs Flashcards
What are primary lymphoid organs?
“Primary Lymphoid Organs are the major sites of lymphopoiesis, the generation of lymphocytes. Here lymphoid stem cells differentiate into mature functional lymphocytes. The primary lymphoid organs are the thymus and bone marrow.
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What are secondary lymphoid organs?
“Secondary lymphoid organs provide an environment in which lymphocytes can interact with antigen and with other lymphocytes: they are the sites at which antigen, antigen presenting cells and mature lymphocytes come together to initiate an adaptive immune response. They have special vascular adaptations to recruit lymphocytes from the blood. Secondary lymphoid tissue includes the spleen, lymph nodes, and mucosa associated lymphoid tissues (MALT).
Describe the thymus
“The Thymus is bi-lobed in mammals, located in the thorax.
Each lobe is organised into lobules and in each lobule are the histologically defined regions of cortex and medulla. The cortex contains the immature thymocytes, some of which are selected to become mature thymocytes in the medulla
“There is a great deal of cell death of developing lymphocytes in the thymus and only a small percentage of the cells (about 5%) exit the thymus into the peripheral T cell pool. The mammalian thymus atrophies with age and areas of active T cell production are replaced with adipose tissue, however the total size of the mature T cell pool does not decline substantially with age.
Describe the bone marrow
“In the foetus, the liver is an active prior to most bones becoming active sites of production. In later life active sites may include spongy regions at the end of long bones; also vertebral bones, sternum, ribs, flat bones of the cranium and pelvis. Bone marrow produces is stem cells and B lymphocytes. Those stem cells destined to be T lymphocytes migrate to the thymus throughout life. For B cells, differentiation is centripetal with the stem cells under the bone and the most mature phases of the B cell pathway found nearer the centre of the marrow”
Describe lymph nodes
“are 1-15 mm across, are round or kidney shaped and have an indentation at the hilus where the blood vessels enter and leave the node. Lymph arrives at the lymph node through several AFFERENT vessels and leaves through one EFFERENT vessel at the hilus. During passage of lymph through the node there is removal of particulate antigens by the phagocytic cells and then this is transported to the lymphoid regions of the node. The cortex is a B cell area and the paracortex is a T cell area.
Describe the spleen
“The spleen contains 2 main types of tissues, the red pulp and white pulp. The red pulp acts as a general filter for blood and the white pulp is the lymphoid tissue and constitutes the major initiator of responses to blood-borne antigens. Around the central arteriole are concentric areas of lymphoid tissue = the periarterial lymphatic sheath (PALS). The region nearest the arteriole is a T cell zone. Periodically, there are B cell follicles, either primary or secondary and around this is the marginal zone which seems to be the primary site of entry of B and T cells into the white pulp.
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Describe MALT
“aggregates of lymphoid tissue which do not have a tough outer capsule. They are found especially in the lamina propria, and sub-mucosal areas of the gastrointestinal, respiratory and genito-urinary tracts. Typical examples of MALT are tonsils and appendix. Other examples include Peyer’s patches (see below). These are organised regions of lymphoid tissue found in the wall of the gut.
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What is lymphocyte recirculation?
“B and T lymphocytes that have matured in the bone marrow or thymus respectively and which have not yet encountered antigen are called naïve lymphocytes. These cells circulate constantly from the blood into the secondary lymphoid organs, and leave the vasculature through a specialised section of the post capilliary venule known as the high endothelial venule (HEV). They move from the lymph node to the lymphoid vessels and eventually return to the blood via the thoracic duct. In the presence of an infection those cells which recognise the infectious agent are held in the lymphoid tissue where they proliferate and differentiate.
How are T cells excavated into lymph nodes?
Rolling, activation,adhesion,trans endothelial migration
Rolling
As cells move across the blood stream, a specialised molecule called selectin is used as a cell cell adhesion factor, which binds the endothelium to the cell. Selectins can be expressed on the endothelium surface or the extravasted cell’s surface.
o Activation
Cytokines attract the cell closer to the endothelium of the vessel. It causes integrins to be activated on the surface of the endothelium or cell. Integrins change conformation
Adhesion
Integrins are changed from a low affinity conformation to a high affinity conformation and bind to molecules on the endothelium and remain stationary on this part, despite blood flow. They then migrate into the tissue, in this example, the lymph node via transendothelial migration
What is a high endothelial venule?
To get from the blood to the lymph, a specialised cell, the high endothelial venule, allows the T and B cells to leave the circulation, to migrate to their positions into the node. Characterised by chemokines
Describe the structure of the lymph node
Lymph enters via the afferent and exits via the efferent lymphatic vessels
B cells are stores on the outside of the lymph node, in lymphoid follicles
T cells are stored separately, towards the middle of the lymph node
Germinal centres are masses of rapidly proliferating B cells during an immune response. During an infection, germinal centres are seen and these are a sign of distinction between the infected and healthy state
What is GALT
gut associated lymphoid tissue (GALT)
The gut has a lot of lymphatic coverage
There are areas of lymphoid tissue called Peyer’s patches. They have aggregates of lymphocytes
Above these are M cells, which sample the contents of the guts for antigens. They uptake any antigens and give them to lymphocytes in the Peyer’s patch.
Peyer’s patches have germinal centres during infection
Describe the cutaneous immune system
Keratinocytes are a barrier
There are lymphocytes in the epidermis (intraepidermal lymphocytes)
Langerhans cells are also there, serving a similar function to M cells in the gut
All the above cells are in the epidermis
The lymphatic system underneath the epidermal layer s there to provide drainage and immune coverage
Antigens can go to the lymph nodes for presentation to the lymphocytes
Dermal lymphocytes can also detect antigens
How can CD markers differentiate lymphocytes?
different cell types are classified by recognition of sites by antibodies. The antibody sticks to
the specific cell surface protein it targets. The antibodies cluster around the cell, which expresses
their specific protein. These clusters are used to differentiate between different cell types
Used to discriminate between cells of the haematopoietic system
More than 300 CD markers
Describe T cell characteristics
T lymphocytes all express CD3. They are CD3 positive (CD3+)
There are two types of TCR, αβ (90% in blood) and γδ (10%).
Two thirds of the αβ express CD4 and the other third express CD8
CD4+ T cells = T helper cells, regulatory T cells (secrete cytokines)
CD8+ T cells = cytotoxic T cells (lyse infected cells, secrete cytokines)
Thymic output declines with age
T cells only recognise processed antigen at the surface of another cell using their TCR. The antigen is presented by an MHC molecule
