Fresh research published in Science seems to hint at a topical anti-allergic effect for cannabinoids. The trial was done in mice using an artificial allergen, and the research proceeded in two steps: first, the scientists found that mice genetically engineered to lack cannabinoid receptors were much more sensitive to environmental allergens like nickel. I suspect the mice were modified as part of Dr. Karsak's ongoing obsession with understanding the role that mammalian cannabinoids (which occur naturally in our bodies, as well as in mice -- one example is anandamide) play in a variety of processes, from atherosclerosis, to generalized inflammation, to mood.
Anyway, Karsak and the rest of the research team decided to take things a step further, and see if cannabinoids from Cannabis itself could reduce skin sensitivity in normal mice. So they cooked up some kind of marijuana salve and applied it to mice that had been exposed to a synthetic allergen (2,4-dinitrofluorobenzene). The results: a 50% reduction in inflammation and swelling. While the mechanism of action is still unclear, preliminary evidence seems to point to cannabinoids' role in modulating the expression of genes that code for pro-inflammatory compounds like histamine. I'd be curious to hear more about this: it could help explain how effective marijuana has been in curbing GI and upper respiratory inflammation (colitis, asthma, e.g.) in previous studies, some of which were in humans.
One final note of wisdom from Roman Rukwied, a pain and inflammation researcher: "We are far before the day when we could say 'oh, I have a nickel allergy. I will smoke marijuana and I won't have it anymore'," he says. "That is definitely not the case." Fair enough.