It's totally possible Jan, you're immune system could react to one allergen and not the other, they are certainly different at the molecular level, now brace yourself for more than you ever wanted to know about allergic reactions. Assuming you are having some type of allergy (and that this is not some other effect related to the effect of a toxin), it is triggered by the action of a unique protein (IgE) that is a component of your immune system, exposure to any allergen causes the production of IgE that will bind only to the protein etc it was made in response to, or to closely similarly structured allergens, when it binds it sets off a cascade of complex events within your immune system, normally this is no big deal, it's a means to protect you. An allergic reaction simply stated is a case of your immune system running wild, it can be anything from mild irritation to fatal anaphylaxis depending on the individual, fortunatly this is rare, but we all know of people who need to carry an epinephrine hypo to use in the event they are exposed to allergens that could be fatal (bee stings, certain food stuffs, etc.) The previous poster that mentioned multiple exposure can lead to more severe reactions was referring to folks like this that produce more and more IgE etc with each exposure, the more you have the more severe the reaction basically, most of us don't produce more, otherwise anyone with hay fever would be dead in short order. Nevertheless, I ain't gonna see how much IgE I have developed towards rabbitfish venom by challenging my immune system again. BTW, my immunology is 30 years old, so I will add the disclaimer that this may all be BS....
