I'm allergic to instant coffee. It's nothing to worry about in the grand scheme of things, and it's easily avoided since I can drink ground coffee with impunity. But it seems that there is something in instant coffee to which I am allergic, that is not present in filter coffee, or just possibly, perhaps, there is something absent from instant coffee that inhibits my allergic reaction.
The reaction itself is quite characteristic, is associated only with instant coffee, and is one I have only come across in one other context: If I drink instant coffee on a daily basis, after about two weeks the skin on the palms of my hands starts to become dry and flaky. My fingerprints disappear, and my hands start to itch. At its extreme, I can see little pinhead-sized white spots under the skin, that, when they reach the surface erupt into areas of dry skin. This is accompanied by an increase in the sweat present on my palms, and an unpleasant, almost painful itch that is only relieved by dunking my hands is the hottest water I can stand.
Elsewhere: apparently, competitive cycling can bring it, or something very similar, on in certain people. I believe that it's also known what metabolic pathway is disturbed, but it's something I haven't really looked into.
I discovered the cause of the reaction quite by chance, after having put up with it for 20 years or so. (Antihistamine and other creams never really worked). I got fed up with the disgusting instant coffee with milk powder from the machines at work, that I drank simply because I was addicted to the stuff. So I weaned myself off caffeine, and miraculously shortly after, my itchy hand symptoms were greatly alleviated. It took homeopathic sulphur a little time later, to clear the problem completely, and I really don't care if homeopathy doesn't work because it can't. It works for me, even if it's completely down to a placebo effect.
I used to have a standard espresso machine, but I got fed up with the way the ground coffee would spill whenever I made a cup, so I invested in a Nespresso, with which I am quite content: no allergic reaction to domestic Nespresso. So I got a Nespresso Pro machine for the gîte as well.
Now Nespresso Pro uses different-shaped coffee capsules from the domestic machines. Apparently, it's to stop people nicking the capsules from work to use at home. And here's the thing. When I drink the Nespresso Pro coffee, I start to get a mild allergic reation to the coffee after a while, just like I used to with instant. Nestlé tell me there is no instant coffee in the Pro capsules, but I am not so sure. Or perhaps it is a colourant, or something. Perhaps even some chemical residue due to the packaging.
So there you have it. Got itchy hands? Look at your coffee consumption. Switch to filter for a while, just to see. Good luck.