Lymphoid tissues Flashcards
What are primary lymphoid tissues?
Sites where new lymphocytes are made - lymphopoesis
What are the anatomical sites of haematopoiesis in a foetus v an adult?
Foetus - all bones and the liver and spleen
Adult - flat bones, vertebrae, iliac bones and ribs
End of long limb bones
What are the primary lymphoid organs?
Bone marrow
Thymus
Foetal liver
What 2 selection processes do T cells undergo in the thymus?
Negative selection - does it detect self antigens
Positive selection - can the T-receptor signal
How does the thymic output change with age?
Thymic output declines with age
thymic involution also occurs (the shrinking and structural change of the thymus with age) and functional tissue gets replaced with fat.
How does number of peripheral T cells change with age?
What does this make older people vulnerable to?
Stays the same
division of mature T cells outside the central lymphoid organs
you get less variety (fewer cells from new lineages)
older people are more vulnerable to new strains of pathogen as they don’t have the primary site for positive and negative selection of T cells
How does number of peripheral T cells change with age?
What does this make older people vulnerable to?
Stays the same
division of mature T cells outside the central lymphoid organs
you get less variety (fewer cells from new lineages)
older people are more vulnerable to new strains of pathogen as they don’t have the primary site for positive and negative selection of T cells
What happens to the thymus and bone marrow during infection?
No change
Increased white cell production
What are secondary lymphoid tissues? Give some examples
Secondary lymphoid organs are where lymphocytes can interact with antigens and other lymphocytes
Spleen, draining lymph nodes, appendix, mucusa of the GI, Epithelial barriers
Why do we have a lymphatic system?
More fluid is filtered than is reabsorbed during tissue perfusion
This fluid somehow needs to be returned to the systemic circulation.
This is achieved via the lymphatic system
What are lymph nodes?
Highly organised, encapsulated structures located at the points of convergence of vessels of the lymphatic system, and hence interrupt the vessels along their course.
What is the immunological function of the spleen?
It collects antigen from the blood and is involved in immune responses to blood-borne pathogens
Lymphocytes enter and leave the spleen via blood vessels.
Describe the organisation/structure of a lymph node
Lymph enters via the afferent vessels and leaves via the efferent vessel.
B cells and T cells enter and leave via the systemic circulation
Consists of an outermost cortex and an inner medulla
The cortex is composed of an outer cortex of B cells organized into lymphoid follicles and of adjacent areas made up mainly of T cells.
What is a Peyers patch? Describe its function
contains numerous B cell follicles with germinal centres -
areas between follicles are occupied by T cells
Antigen collected from gut by specialised epithelial cells - M cells
dendritic cells residing within the peyers patch present antigen to T-lymphocytes
Effector lymphocytes travel into the bloodstream to carry out their function
Describe the structure of the spleen
mainly composed of red pulp – site of red blood cell disposal
Lymphocytes surround the arterioles running through the spleen, forming isolated areas of white pulp
The sheath of lymphocytes around an arteriole is called the periarteriolar lymphoid sheath (PALS) and contains mainly T cells.
Lymphoid follicles occur at intervals along it, and these contain mainly B cells.
What tissue forms the first line of defence against infection? How does the immune system protect it?
Epithelial/mucosal surfaces
form a physical barrier to pathogens
Protected with extensive system of lymphoid tissues known as mucosa-associated lymphoid tissues (MALT)
Skin protected by the cutaneous immune system
Give an example of a MALT (mucosa associated lymphoid tissue). List its locations in the body.
Gut-associated lymphoid tissues (GALT)
Includes the tonsils, adenoids, appendix, and specialized structures in the small intestine called Peyer’s patches
What is a germinal centre?
Anatomically restricted site where B cells undergo mutation and selection to generate high affinity antibodies
What cells are part of the cutaneous immune system?
Langerhans cells - type of dendritic cell Keratinocytes Tissue resident macrophages T-lymphocytes Intraepidermal lymphocytes
Summarise the principle by which the lymph nodes, spleen, and MALTs all operate.
Trapping antigens and antigen-presenting cells from sites of infection in order to present antigen to migratory small lymphocytes, thus inducing adaptive immune responses
Where do mature lymphocytes go after they are produced?
They enter the bloodstream and migrate to populate and continuously circulate through the peripheral lymphoid tissues such as lymph nodes, spleen, and mucosal lymphoid tissue of the gut, the nasal and respiratory tract, the urogenital tract, and other mucosa.
How do lymphocytes actually get inside lymph nodes?
T and B cells enter the lymph node via specialized blood vessels called high endothelial venules (HEVs) that are found in T-cell zones.
Entry of lymphocytes into lymph nodes occurs in distinct stages that include initial rolling of lymphocytes along the endothelial surface, activation of integrins, firm adhesion, and diapedesis across the endothelial layer into the paracortical areas, the T-cell zones.
Regulated by a coordinated interplay of adhesion molecules and chemokines
How does the antigen go from the site of infection to the peripheral lymphoid organs to activate lymphocytes?
T cell activation occurs via encounters with dendritic cells that have ‘picked up’ antigens at sites of infection and migrated to secondary lymphoid organs.
Free antigens can stimulate the antigen receptors of B cells, but most B cells require ‘help’ from activated (helper) T cells for optimal antibody responses.
Dendritic cells are known as professional antigen presenting cells - range of migratory and tissue resident variety.
Describe in more detail the transport of lymphocytes and antigens in the lymphatics and bloodstream
Free antigen/antigen-bearing dendritic cells travel from site of infection through the afferent lymphatic vessels into the draining lymph nodes
Activated lymphocytes proliferate and differentiate
most leave the lymph nodes as effector cells via the efferent lymphatic vessel to return them to the bloodstream, which then carries them to the tissues where they will act.
Naive lymphocytes that do not recognise their antigen also leave through the efferent lymphatic vessel and are returned to the blood,
they continue to recirculate through lymphoid tissues until they recognise antigen or die.
