According to new research by scientists from the Institute of Food Research in Norwich and the Medical University of Vienna, the equivalence of an animal food protein to a human protein decides whether it can cause allergy.
So far, all proteins have been believed to have the potential to become allergens, but the study found that the ability of animal food proteins to act as allergens depends on their evolutionary distance from a human equivalent.
"This explains why people who are allergic to cow's milk can often tolerate mare's milk but not goat's milk", Dr Clare Mills of the Institute of Food Research said.
"Proteins in horse milk are up to 66 percent identical to human milk proteins, while known allergens from cows and goats are all less than 53 percent identical to corresponding human proteins," he said.
"Overall we found that only an animal food protein that is less than 54 percent identical to a human equivalent could become allergenic," he added.
Generally, the most common animal food allergens in adults are fish and seafood, while cow's milk and hen's eggs are common causes of allergy in infants.
From the analysis of the study, the researchers found that the majority of animal food allergens could be classified into one of three protein families.
Tropomyosins, being the most important family are proteins found in muscle tissue.
"Tropomyosins in mammals, fish and birds are at least 90 percent identical to at least one human tropomyosin and none have been reported to be allergenic, " Dr Heimo Breiteneder of the Medical University of Vienna said.
"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", he added.
The second largest animal food allergen family was formed by EF-hand proteins.
Caseins, all mammalian proteins from milk, were formed as the third animal food allergen family, those in birds and mammals are not allergenic, while those in frogs and fish can cause allergy.
The researchers also analysed milk from rabbits, rats and camels as well as sheep, goats, cows and horses.
"Animal food proteins lie at the limits of the capability of the human immune system to discriminate between foreign and self proteins", Mills said.
"Immune responses to some animal food allergens such as the invertebrate tropomyosins, run close to becoming a form of autoimmune response and this needs to be considered when developing allergy therapies," he said.