In case you were wondering about skin testing, here is an example. In the top picture, this is testing for 1) milk, 3) egg, 25) oats, 54) soy, and a (s) saline and (h) histamine.
The second picture shows + results to milk and egg. Oat was negative. Soy was also a small +.
The histamine is + but it should be because it is the control and shows that there is a normal reaction to histamine. When histamine is released, it can cause itching, redness, hives, etc. You are intentionally pricked with histamine to make sure it is + to ensure that the test results are valid. For example, if you were taking zyrtec regularly (a histamine BLOCKER), the histamine skin test would be negative. It would also mean that you may not turn up + for foods that you may be + to making the results invalid.
in contrast, the saline should be negative because a person should not react to simple normal saline. There are instances where people do react to saline, though. Some people are "dermatographic" which means that a whelp may develop simply by pricking skin. For example, if an object is scraped on your arm in a straight line, you will develop a "straight line" whelp. This would mean that you are dematrographic and it is unlikely that skin testing would work for you - you would probably react to everything although they would not be considered +.