In a recent post on the National Eczema Association website I explored allergy testing for eczema. Although onset is rapid, I wouldn't classify my reaction to hot pepper as an allergy. My symptoms are typical, except for the itchiness. And there is a well-known molecular explanation for how hot pepper gives you a burning, painful sensation, especially when you get some on your skin.
The active ingredient in pepper is a molecule called capsaicin. According to Wikipedia,
The burning and painful sensations associated with capsaicin result from its chemical interaction with sensory neurons. Capsaicin, as a member of the vanilloid family, binds to a receptor called the vanilloid receptor subtype 1 (VR1)... By binding to the VR1 receptor, the capsaicin molecule produces the same sensation that excessive heat or abrasive damage would cause, explaining why the spiciness of capsaicin is described as a burning sensation.I'd assume that when you eat hot pepper, you don't break down capsaicin; instead, most of it gets distributed all over your body, where it makes you feel hot, and triggers symptoms of inflammation. Great! The pepper-induced inflammation is likely generating signals in neurons that carry itch sensation to the brain.
...Essentially, the body inflames tissues as if it has undergone a burn or abrasion and the resulting inflammation can cause tissue damage in cases of extreme exposure, as is the case for many substances that cause the body to trigger an inflammatory response.
But there's no explicit documentation of this in the scientific literature. When I searched for research connecting capsaicin and eczema on PubMed, the NIH archive, I found virtually nothing.
Instead, I was intrigued to discover on Wikipedia, PubMed, and the internet in general that capsaicin, far from being fingered as an evil trigger of eczema, is often touted as a TREATMENT for eczema. There are capsaicin-containing creams that you are supposed to rub on your skin to relieve itch. The theory, I guess, is that the burning pain overwhelms the itch neurons. I get that.
Unfortunately you're not supposed to rub capsaicin on open wounds. And with eczema, your skin always has open wounds. Even worse, the one clinical study I found showed that capsaicin cream relieved itch only for people who did not suffer from atopic dermatitis. Capsaicin appears to control itch that is caused by histamine; but chronic eczema itch is not caused by histamine, and so topical capsaicin does nothing for it.
In short, if you have eczema, hot pepper is not your friend, in any form. I love my hot sauce, but I can only look on it as a vice, not a virtue.