Primary lymphoid tissues: thymus Flashcards
Where is the thymus located?
In the lower part of the neck, and deep to the sternum, slightly inferior to the larynx.
Where do T cells complete maturation?
In the thymus.
When is the thymus at its largest?
It enlarges through childhood when it is most active, but after puberty it begins to reduce in size and function.
During maturation, why do most developing T cells die in the thymus?
They fail to produce a T-cell receptor that is useful to the immune system.
What does positive selection select for?
Only those T-cells capable of recognising self-MHC molecules and peptide.
What presents antigen to T cells on MHC-I and MHC-II?
Cortical epithelial cells.
What happens if the T-cell successfully recognises the MHC-protein complex?
It receives “survival signals” which prevents it from entering apoptosis.
Where do surviving T cells advance to?
The medullary region.
What happens if the T cell does not recognise the MHC-protein complex?
The T cell dies by apoptosis, after 4 days, 95% of T cells that make it to the thymus fall to this fate.
What happens in the medullary region?
Negative selection eliminates those T cells which would recognise self-peptide and therefore be dangerous to the body.
What do dendritic cells do in the medulla?
They present self-antigen to the T cells on MHC-I and MHC-II.
What happens to the cells that do bind to the self peptides presented to them by the dendritic cells?
They are given a death signal and they are told to undergo apoptosis.
What happens to T cells that don’t bind to the self antigen presented by the dendritic cells after four days?
They are deemed as useful and leave the thymus, entering the blood stream.
