Nutmeg is a spice in the Myristicaceae family of plants. The spice is made from the brown seed kernel inside the fruit pit of the plant. The spice mace is made from the same plant, but is from the red or purple lacy covering on the pit. If allergic to one, you would likely be allergic to the other.
Nutmeg is NOT a nut, so does not have to be avoided if you are allergic to tree nuts or peanuts.
Nutmeg contains isoeugenol which is a chemical compound which can cause contact allergies – it is found in nutmeg oil which can be added to foods and cosmetics for flavour or fragrance.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Nutmeg.php
The allergy resources page for nutmeg covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with nutmeg.
There is also regularly updated links to useful websites specifically for nutmeg allergies and the latest research papers on the topic.
If you have multiple allergies and are interested in how they are linked or want to find out more about food allergies outside the usual top 14 then you can follow me on Facebook or Twitter, where I share a food allergy card every day of the week.