If you are cooking for someone with an allergy, always read all the labels of the ingredients thoroughly before you use them to check that they are OK and don't contain food traces that might cause a problem (when you start really researching this you discover many seemeingly innocent foods could contain allergens because of the factory they’re processed in). You might even want to get your guests to check the ingredients too, just to make sure.
You’ll also need to ensure your surfaces, utensils. baking trays etc. are scrupulously clean before cooking in case there are a few stray crumbs that might cause a problem.
Beware also of jars of jam, pickle (and similar), that could be contaminated with butter, breadcrumbs or peanut butter, or whatever.
If you want any more advice, check out the Allergy UK website, which has many informative factsheets about dealing with food allergies (Click here)