Any of a class of naturally occurring complex combinations of amino acids (containing carbon, hydrogen, oxygen, nitrogen, usually sulfur, occasionally phosphorus) that are essential constituents of all living cells, being responsible for growth and maintenance of all tissue, and the essential nitrogenous constituents of the food of animals; some individuals mount an excessive immunological response (allergy) when exposed to specific protein allergens to which they have developed a Type I hypersensitivity. (e.g. latex protein allergens.)