Do they expect you to WANT to eat foods that give you allergies?

They’re making fun because they don’t believe there’s such a thing as a lettuce allergy. I’m not buying it. Just get the burger with lettuce and discreetly slip the lettuce to your dog, assuming your dog is not also a ridiculous hypochondriac.

You can try educating them on what would happen to you if you ate it. But don't expect that to change anything. People can be really stupid about other people's food allergies,. The most common mistake is that they think saying "I'm allergic to that food" is just another way of saying "I just don't like that food." My own sister occasionally compares my food allergy to her personal distaste for musicals. Every time she does this, I point out that the difference is that if someone made her watch a musical, she wouldn't actually throw up or die. She really and truly does know that it's not an accurate comparison, but, for some reason, she thinks it's funny to equate her personal preferences to my involuntary physical reaction to certain foods.

you're allergic to lettuce?

It's odd to be allergic to lettuce... Do you know what exactly causes it? Because you might be allergic to one specific component and so be allergic to other food with it since lettuce is composed at 94% of water...

Since lettuce is almost 95% water. I’d be amazed if you were actually allergic to lettuce. You’re most likely allergic to whatever pesticides they use than actual lettuce but whatever. Either leave it on and see what happens or take it off and listen to them complain. Your only two choices here