Every once and an annoying while, I will accidentally eat something I am allergic/intolerant to and get sick. This usually happens in public places, since I don't cook things I can't eat at home. Inevitably, someone will say, "but you just had a few bites, you're really sensitive."
Sensitive? That's a dumb thing to say. If someone ate just a little bit of arsenic and got sick, would you say, "gee, you're really sensitive to poison!" Or if they drank a bit of bleach and burnt all the lining of their esophagus, I could just see a doctor now saying, "the patient is really sensitive to bleach...."
It's a little extreme, but the point is the same. The fact is while food allergies are just caused by harmless food you normally eat, to a person with an allergy, their body reacts as if they've been poisoned. So for these people, some foods ARE poison, even deadly.
I don't want people to mix up being sensitive (or the implied weak or fragile) with a food sensitivity, which has a scientific definition and physical response. I have food sensitivities, but I am not sensitive to food, nor does the food make me sensitive. Non-allergic food hypersensitivity is the proper scientific term for "food intolerance." My body cannot tolerate or digest certain foods. I am intolerant.
I'll explain what happens to me. When I accidentally eat shellfish, even if it's just a little, my face flushes and I start to feel hot all of a sudden. I start burping (dyspepsia) and feeling odd. Within an hour, I am doubled over with intestinal cramps and diarrhea. If I can catch what happened fast enough and get some Benadryl, it's not as bad a reaction. However, I've had the effects of just a small amount of shellfish last all night to all weekend. It's no joke and it's not fun. My father was intolerant to shellfish and so is my son, unfortunately.
The difference between an intolerance and an allergy is an intolerance usually has to do with lacking the digestive enzymes to properly digest certain proteins, whereas an allergy is the body over-reacting to what it believes is an invader. Intolerances are more likely to affect the gut or mucous membranes (sinuses included) and allergies are more likely to cause anaphylaxis - which just means more than one reaction occurs at the same time. Contrary to common belief, it does not mean the person swells up and can't breathe (though that can be part of anaphylactic shock). All it means is more than one part of the body reacts simultaneously, so it could be diarrhea and hives. Or nausea and runny nose.
So I hope that clarifies things a bit and also how important it is to treat these things seriously. When people with food allergies or intolerances make special requests not to include a food, it's not because they don't like it or are picky eaters trying to cause problems. It's so they don't have their whole weekend ruined on the toilet or actually die.
Note: While searching for images, I found this controversial article on how food allergies can be good for you (their opinion, not mine!) Read it if you're interested.