Not the fragment. Not the plate.
The one surface the two make together.
An antibody sees one thing. A T cell receptor always sees two — a fragment, and the plate holding it.
So the same fragment on a different plate is a different thing. That one rule, found in 1974, explains transplant rejection and checkpoint-inhibitor resistance in the same breath.
These twenty lessons stay on that single interface. You count VDJdb's 181,411 rows and 24,500 IMGT/HLA alleles yourself, and confirm that what differs between people sits in the groove alone, while the loop CD4 and CD8 hold on to barely moves — and that more than half of the table piles onto a single plate.
How you'll learn
We take all 5,839 HLA-A alleles and compute entropy position by position. The groove (α1·α2) reads 0.21 and 0.25; the platform (α3) reads 0.08. What differs between people is only the face the TCR touches.
Positions 223–229 read DQTQDTE with entropy 0.011. Across 19,000 alleles it barely moves — which is why one CD8 works on every person's class I.
57% of class I records sit on HLA-A*02:01 — a plate carried by only about a quarter of people. Lesson 19 counts what that mismatch costs.
A TCR binds ten thousand times weaker than an antibody. Dwell time, not strength, decides the answer — which is how a single residue turns an agonist into an antagonist.