Virtually all cheeses available for purchase at supermarkets and cheese shops are as close to gluten-free as possible, which means they all contain an undetectable level of gluten. The main ingredients in cheese are milk, rennet (a milk-curdling enzyme), and bacteria (which ferments the milk).
When practicing a gluten-free diet, one must always be wary of gluten cross-contamination, as well as when other ingredients are added as flavorings into cheeses. A new trend of beer and spirit-soaked cheeses is emerging, such as Irish cheddar infused with stout beer or Irish whiskey. Both of these add a dangerous level of gluten to the cheeses they flavor. In these instances, it is highly recommended to those suffering from gluten allergies to avoid consuming any of these types of cheese.
Additionally, people suffering from gluten allergies or celiac disease have been concerned about certain types of blue cheeses containing gluten because the bacteria classically used to make them, Penicillium roqueforti, is traditionally grown on rye, malt, or wheat-based dextrose, all of which contain gluten. A study in 2009 by the Canadian Celiac Association tested a group of blue cheeses for gluten, and found that the levels were all less than one part per million, which is an undetectable level. Still, people who have celiac disease should always proceed with caution, so always ask your the salesperson for as much information as possible about the origins of a product you're considering for purchase.