Hi, hope he is ok after his jabs

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Your doc is true in saying that if you are allergic to one nut, you might not be allergic to all. With some people its brazil nuts, but none others!.
Also true that your body will not react to something the first time, only subsequent times when your body has built up antibodies to it. So first time eating something, no reaction, next time .. perhaps an allergic reaction. However this is based on the fact that you would not have had something unknowingly. Such as using nuts for example,nut derivatives can be found in things even though you have never actually eaten nuts.
Doc is also correct you develop an allergy being exposed to something whether it be food or other things, and only when you come in to contact with them.
With regard to the fish, I have never heard of this at all, and no idea why not having it whilst pregnant would make any difference to being able to digest it... weird.