Can I give a person an allergic reaction at a very specific spot in a tissue? And if so, how accurate can i get?
Depends on the kind of allergic reactions you want to create. If your target tissue is skin epithelium sure you can.
That's actually the way the skin allergy test is all about: The physician tries a wide variety - last time I did it were 28 well known kinds - of substances in liquid form (drops). If an allergic reaction is caused, the physician takes note. Even very small quantities can cause allergic reactions.
These tests are usually made in a secured environment (physician's office, hospital, etc) at a very low level which most of the times, if result is positive will cause just an irritation. The level of irritation will be self-explanatory, so even a non trained patient can understand what is going on.
Not sure if that what you were asking though.