Proteins are fundamental building blocks in living organisms. Anything from cell walls and other structures, to enzymes and other functional chemicals are made up of proteins.
Proteins are large molecules made up of amino acids, and arranged in a very specific way. This specific size, shape and “texture” makes each protein unique and highly recognizable.
Many proteins are specific to a single species or small group of species.
For example, gluten is a protein – actually a class of proteins. Nearly identical gluten types occur in wheat, barley and rye. A similar type occurs in oats, and different gluten proteins occur in other grains.
Because proteins are so specific, and the immune system is so good and differentiating one from the other, someone with a an allergy to, say, wheat gluten, will probably not be allergic to oat gluten – although they may be allergic to that related protein as well in some cases.