How would we have an immunoglobulin E that reacts to an alien we have never encountered before?
Your body produces vast numbers of antibodies with subtle variations in their binding structures generated through a mechanism called Somatic Hypermutation - including many that react to no known analog in nature (at least on this planet).
As well, there's more than one pro-inflammatory pathway that can trigger the excess of Histamine required for a lethal reaction - though most are perceptibly slower.
Perhaps by an anomalous Cytokine response triggered by alien proteins that were (by a mathematically improbable coincidence) similar to those of an earth parasite? You'd only have to inhale a couple of pollen spores for an immune response to begin, so naturally consuming a portion of some xenofood would make it much worse as you'd have lots more proteins to react to & only emesis to remove them from your stomach (not to mention the damage to your intestinal flora, which you need to absorb several essential nutrients).
It might not be as spectacular as the Type 1 Hypersensitivity that someone allergic to peanuts would experience on eating some peanut butter, but the end result would be just as unpleasant, and deadly.