A key difference between allergy and intolerance is that allergic reactions have an immunological basis, while those with other causes are classed as intolerances. Intolerance of wine is far more common than true allergy to wine or grapes. The most common allergens, chemicals capable of causing an allergic reaction in humans, are proteinaceous compounds. Among possible allergens in wines are traces of the natural proteins not precipitated and removed with the dead yeast cells after fermentation, and traces of proteins from fining agents used to clarify and stabilize the wine. Pollen-food allergy syndrome affects those severely sensitized to pollens which cross-react with similar proteins in fruit skins, causing oral itching. Grape allergy appears to be largely found around the Mediterranean, and chitinases may be the major allergen. Even traces of wasp venom found in wine have been reported to cause reactions. The biogenic amines histamine and tyramine produced by lactic acid bacteria during malolactic conversion and present at higher levels in red wines, have been implicated in triggering headaches and red-wine intolerance.