T Cell Proliferation- Any experts?

[Mr. P.J. Foley]

I am currently setting up some T-cell assays to identify HLA epitopes. I
am about to order some peptides of between 15-23 residues lenght. These
come at 70%, 80% and 95% purities from the manufacturers.

Does anyone have any suggestions about the optimum purity to use- I know
the higher purity preps cost more- is this worth the difference in terms
of results etc?

Also does anyone know if the sizes of my peptides are suitable- should
they be smaller? Is there a maximum length that will give good results?

Thanks in advance,

Dr. Hari Veeraraghavan, National Heart & Lung Institute London.