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[Inside the thymus]

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Our immune system fights
infections and cancer cells,

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with a type of white blood cell,

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the T cell.

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T cells are made in an organ
called the thymus.

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Immature blood cells are made
from blood stem cells in the bone marrow,

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travel through the bloodstream
to the thymus.

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Inside the thymus, over the course
of approximately 2 weeks,

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a network of different cells
impart chemical signals

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that affect the fate
of the immature T cells.

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They're guided through
a process of development,

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selection,

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and also elimination
to become mature T cells.

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Firstly, the immature T cells
are instructed to develop receptors.

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At the next stage,
those cells with receptors

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that can respond to the type of proteins,
the ones useful to our immune system,

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are signaled to survive.

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The rest are eliminated.

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After this stage,
some of the developing T cells,

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they recognize proteins
from the healthy cells of our own body

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which can be dangerous
and cause auto-immunity,

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where they may target
those cells and tissues.

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In the final stage,
these cells are eliminated.

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The remaining now matured T cells,

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which have passed
through the selection process,

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enter the bloodstream
and become part of our immune system.

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In fact, as little as one percent
reach this stage

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and are now ready to protect
against harmful infection and disease -

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but not the healthy cells of our own body.

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From a young age our thymus
degenerates, getting smaller.

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Throughout our lives,
fewer and fewer T cells are made,

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reducing our protection against
new infections and cancer,

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especially, in old age.

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Finding ways to regenerate the thymus

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could help to boost the immune system
when this happens

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and could also lead to treatments
for a number of conditions

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affecting both the thymus
and the immune system.