Good discussion going on guys. I have a few books on tobacco at home, but I am at work so I can't go too in depth knowledgeably, but I'll throw this bit of info out there.
__________________
Virginia Tobacco
Virginias are the king of tobaccos and the very crop our country was built on. As you may have already deduced from the name, it was originally grown in well-known VA Plantations. Today, Virginia tobaccos are grown not only in VA but also eastern NC, northeastern SC, and, indeed, around the world. The term "Old Belt" is an appellation and refers to Virginias grown in VA and NC.
Virginias are primarily classified by their color, and while the exact nuance of hue lends itself to a vast array of categorizations, think "red" and "bright". The relative location of a given leaf on its stalk is what ultimately determines its color; the brights come from the middle of the stalk, and the reds originate from higher up to the tips. While all Virginias are sweet, the further a leaf moves toward the lighter side of the color spectrum, the sweeter it becomes, but along with that sweetness comes additional acidity and hints of citrus-like tartness. As we move towards progressively darkening colors, along with an increasing depth of flavor comes a taste that shifts to something akin to baking bread.
______________________________________
The highlighted portion (Blue text) is the part that I'm going to refer to.
Since Red Virginias have a higher acidity content than the darker Virginias, maybe the acidity is what some people are allergic to. Not the plant itself per se, but the acidity that is found in that part of the plant. So in essence, they may be allergic to Red Virginias not because of the plant, but because of the acids that are IN that part of the plant makes them allergic to that specific part of the plant.
Full Disclaimer: I am not an allergy doctor. Heck I'm not a doctor of any type.
Although I do play one at home sometimes ?.