Food allergies can be very serious. The worst reactions medically are the life threatening ones, but even "minor" allergies are no fun for the person suffering from them. You have to be careful what you eat when you have a food allergy. Exposure to the allergen can cause the allergic reaction to get worse over time. Something that was a minor allergy can transform into a major, life threatening allergy without warning, and allergens can come from surprising places.
I also will try to avoid common allergens when making treats for large gatherings. It's not fun to be at a gathering and not able to eat any of the food sitting out! If you've left out common allergens, label the dish. Include the ingredients.
Some people don't have an allergy to the foods, but are sensitive. Sensitivities usually indicate that the person has a difficulty processing the food. This can have unpleasant side effects, so a lot of people with sensitivities avoid the foods in question.
Here are a few tips for how I deal with making allergen free treats when I know someone suffers from them, or if making for a gathering with unknown allergens.
1. Do not use wood utensils. Wood is porous, and can hold on to things you don't think are lurking. Only use hard plastic, glass, or metal utensils. If using plastic, make sure that it is not scratched. Scratches can hide allergens. For people with serious allergies, even the tiniest amount of the thing the are allergic to can cause them to have a reaction.
2. Use metal or glass mixing bowls. Make sure if you are making something that will have a reaction to metal to use a glass bowl instead.
3. If you have to prepare on a porous surface, make sure you clean the surface very well, and cover it with something such as aluminum or waxed paper.
4. Consider having a special set of allergen free bakeware and utensils. If you will be making allergen free treats frequently, this is definitely a good idea. Store them separately from your regular bakeware to avoid cross contamination.
5. Make sure that you understand the allergy of the person you are making the treats for. It is always better to go a little overboard than to risk a reaction, especially anaphylaxis. This link will take you to a website that explains some of the common food allergies, and lists (most) of the triggers.
6. Prepare AND package any allergen free treats before any treats you will be making that contain allergens. Make sure to carry them in separate containers to their destination.
7. If you are going to be giving out or selling the foods (such as at a bake sale) then make sure that you label everything. List EVERY ingredient, even water. If it is allergen free, label what it doesn't have in it (like labeling a treat Gluten Free). Even a hand written note is better than no ingredient list. Some schools are actually requiring an ingredient list for all bake sales, on each individually wrapped goody. If you have a printer, you can make some very cute labels for them. Tie them on with a ribbon, or print on address labels so you can stick them directly to the package.
Please let me know if I forgot anything, and how you make treats that can be enjoyed by everyone in the comments.