So how come some people are allergic to some animal proteins, like those in cow's milk, but not in a similar food item, like horse's milk?



According to new research by scientists from the Institute of Food Research in Norwich and the Medical University of Vienna, whether a particular animal protein can cause allergies largely depends on how related an animal food protein is to a human equivalent of that protein.

For the first time the researchers found that the majority of animal food allergens could be classified into one of three protein families. Tropomyosins, proteins found in muscle tissue, are the most important family.

"Tropomyosins in mammals, fish and birds are at least 90% identical to at least one human tropomyosin and none have been reported to be allergenic. In contrast, the allergenic tropomyosins are all from invertebrates such as insects, crustaceans and nematodes and at most are only 55% identical to the closest human homologue", said Dr Heimo Breiteneder of the Medical University of Vienna.

EF-hand proteins form the second largest animal food allergen family. Those in birds and mammals are not allergenic, while those in frogs and fish can cause allergy. The third animal food allergen family, caseins, are all mammalian proteins from milk. The researchers analysed milk from rabbits, rats and camels as well as sheep, goats, cows and horses.

It appears that if an animal food protein is less than 54% identical to a human equivalent, it could become allergenic. In the case of milk, proteins in horse milk are up to 66% identical to human milk proteins, while known allergens from cows and goats are all less than 53% identical to corresponding human proteins.

Still, while this might explain why an allergic reaction is staged, it still doesn't solve the riddle of why some people have allergies at all, while others can gulp liters and liters of cow's milk without showing any apparent allergic symptoms.