Show Me a Piece of You
Asystole

I hid in a cell with the curtains drawn,
Thought every trace of my visit was gone.
Then little pieces appeared at the door—
The host had a story it wanted to show.

Proteins in the cytosol enter the scene,
The proteasome cuts them into peptides clean.
T-A-P takes peptides to the E-R,
Where class one loads a sample of who we are.
Most nucleated cells show peptides every day,
M-H-C class one puts the sample on display.
An armed CD8 with a matching receptor
Can kill the infected cell, a careful inspector.

One to eight, two to four,
Show me a piece at the cellular door.
Class one, CD8; class two, CD4,
Peptide and M-H-C, that's what they're looking for.
One to eight, two to four—
Show me a piece of you, just one piece more.

A dendritic cell swallowed a piece of my coat,
In an acid compartment my fragments float.
Endosomal proteases cut me down to size,
Class two can carry the next surprise.
Macrophages, B cells join that presenting crew,
Showing what they've taken in on M-H-C two.
A matching CD4 can read the display,
And help direct what happens the following day.

One to eight, two to four,
Show me a piece at the cellular door.

Invariant chain keeps class two's groove reserved,
Till a little CLIP fragment is all that's preserved.
H-L-A D-M helps CLIP move away,
Exchanging it for a peptide to display.
Inside to one, taken-in to two:
Those are the usual routes to keep in view.
Self peptides travel these pathways as well;
A display alone doesn't condemn a cell.

A dendritic cell says, "Here's another way:
Outside material on my class-one display."
Cross-presentation gives CD8 a view,
Of something that another cell went through.
Recognition joins the signals that decide
Which matching T cells multiply and ride.

One to eight, two to four,
Show me a piece at the cellular door.
Class one, CD8; class two, CD4,
Peptide and M-H-C, that's what they're looking for.
One to eight, two to four—
Show me a piece of you, just one piece more.

I hid in a cell with the curtains drawn.
My pieces gave away what was going on.