Researchers at Chinese University on Tuesday said they have developed a ladder system to help children with fish allergies reintroduce the food into their diet.

The researchers said that between 2016 and 2021, they studied 249 children with fish allergies.

Many of them were found to be allergic to the protein β-parvalbumin.

The researchers said some types of dark fish, such as salmon and tuna, have lower levels of the protein in them compared to fresh fish, such as tilapia and catfish.

One of the researchers, Agnes Leung, said she hopes the ladder system can help children with allergies eat some kinds of fish, because the food has nutrients that can't be found elsewhere.

"Fish has very unique nutritional properties, for example, Omega 3 fatty acids are unique to fish, they help a growing child's brain development," she told RTHK.

The researchers said they found that around 30 percent of the children in the study were not allergic to shellfish, such as shrimp and crab.

"The major allergen in fish is parvalbumin, but the major allergen in shellfish is tropomyosin, so they are fundamentally different," Leung said.

"For a lot of fish-allergic patients, they could in fact tolerate shellfish without developing allergic reactions."