It would be interesting to see if anti-histamines(A/H) actually effect wheal size. You would expect them to, but as it has been found that A/H doesn't seem to help POIS much, maybe they wouldn't effect the wheal either.
It´s my impression though that A/H doesn't seem to have much effect over POIS because it only effects the type I reaction, which is only part of POIS for most of us.