9-11 amino acids sounds very short - only 18-22 bits of information. How do we decide something has gone wrong from only an 18 bit slice?
This is also a clue as to why we don't have a perfectly rigid system, as a library of T cells capable of recognising every combination would weigh 600+ kg.
There are 20 amino acids. Each AA in a sequence represents approx 4.3 bits. So 9-11 AA would be 38.7-47.3 bits. Not quite as much as an MD5 hash (128 bits), but still quite a bit of info.