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37 Cards in this Set

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What are the 2 groups of lymphoid organ classifications?
Primary
Secondary
What are the primary lymphoid organs sites of?
B and T lymphocyte maturation
Why is Lymphocyte maturation sequestered in a specific organ from the bone marrow?
-To delete lymphocytes that FAIL to express Ag receptor
-To select for cells that express Ag receptors recognizing self Ag's appropriately
How does an Ag receptor that appropriately recognizes self Ags behave?
-Neither too well
-Neither too poorly
What are the Primary lymphoid organs in humans?
-Thymus
-Bone marrow
Where is the thymus located?
Just above the heart
How does the thymus size change with age?
It gets smaller
What happens when the thymus has to be removed during cardiac surgery on infants?
There is increased susceptibility to infection
What does this tell us about what goes on in the thymus?
It is the site of Tcell maturation.
What is the macroscopic anatomy of the thymus?
-2 lobes
-surrounded by a capsule
-Capsule makes septations
-Divides it into lobules
What is each thymic lobule composed of?
-Outer cortex
-Inner medulla
What cells enter the thymus from the bone marrow?
Thymocytes
Where are thymocytes most highly concentrated in the thymus?
In the cortex (but they're also scattered in the medulla)
What other cell type is found in the thymus?
Epithelial cells
What type of epithelial cells line the internal surface of the thymus capsule and septations?
Subcapsular-Perivascular Epithelial cells (Type I)
What does the Subcapsular epithelium function to do?
Form a sort of blood-thymus barrier to modulate the entry of systemic antigens into the thymus.
Does the subcapsular-perivascular epithelium contribute to the thymocyte development?
It's important for it to be normal, but does not participate in functional interactions.
What type are the Subcapsular perivascular epithelial cells?
Type I
What are Type II and Type II epithelial cells in the thymus?
Cortical epithelial cells
What is special about cortical epithelial cells?
They have long dendritic processes that contact each other and developing thymocytes.
What is found on the surfaces of the dendritic processes of cortical epithelial cells?
Peptide/MHC complexes on bone marrow derived APC's
What interacts with these antigen presentations?
Developing thymocytes' TCRs
What happens if the TCR binds the MHC/Ag complex poorly?
It gets eaten up by cortical macrophages and spit out.
What % of all developing thymocytes fail to bind well and thus meet their gory death?
98%
What happens if the TCR DOES bind the MHC/Ag complex well?
It gets promoted to the medulla
What is the process of promotion to the medulla called?
Positive selection
What are the 3 types of medullary epithelial cells?
-Type IV and V medullary epithelial cells
-Type VI medullary epithelial cells
What is the function of the Type IV and V medulla epithel cells?
Negative selection - present Ag/MHC to TCRs, and if they don't bind with high affinity they survive (not autoimmune).
So what is the function of
-Positive selection
-Negative selection
pos - makes sure they bind
neg - makes sure they don't bind self
What do Type VI medullary epithelial cells do?
Form Hassal's corpuscles and eat the thymocytes with TCRs that were too high affinity binding.
What is the well described clinical pathology of the thymus?
DiGeorge Syndrome
What causes DiGeorge Syndrome?
Failure of the 3rd and 4th pharyngeal pouches to develop
What are the 3/4th pouches important for?
-Thymic development
-Parathyroid development
-Aortic arch development
What is the cause of DGS in 90-95% of patients?
Hemizygous microdeletion of DNA from chromosome 12

12q11.2
Does complete thymic aplasia develop in DGS?
Only in about 1/2% of patients
What are the most common problems in patients with DGS?
Prolonged viral infections, 2ndary bacterial infections, and autoimmune disease.
Is treatment needed for the majority of patients?
no