No more shrinking from shrimps

PRAWNS are considered a delicacy in most parts of the world, but not everyone can enjoy eating them for they can cause severe allergic reactions -- pain in the abdomen, nausea, diarrhoea, hives, choking and even death.

But now, P V Subba Rao and his colleagues at the Indian Institute of Science in Bangalore have identified the offending proteins that cause the allergy. This, says Subba Rao, will enable the scientists to develop vaccines that can prevent such allergies. The research is part of an Indo-USAID project on sea food allergies.

Subba Rao points out that hypersensitivity to shrimp meat is quite common, though, according to him, more people are susceptible abroad than in India.

The offending constituent was a muscle protein -- tropomyosin -- in shrimp meat. This protein, the scientists found, retains its structure even on cooking. The identification of tropomyosin as the main allergen has wider implications, explains Subba Rao, because people with shrimp allergies may be more susceptible to Crohn's disease, also known as chronic inflammatory bowel disease -- an autoimmune disease where patients are allergic to their own tropomyosin.

The scientists tested various shrimp meat extracts by combining them with the blood serum of patients who were allergic to shrimps. A few of the shrimp meat extracts reacted with the blood, paving the way for the discovery of the allergen.

The scientists then chemically cut the protein into fragments, and identified the section of the tropomyosin molecule that causes the allergic reactions. They have synthesised the allergy inducing protein fragment that is also present in related species such as crab and lobster, which is why people allergic to shrimps are usually also allergic to crab and lobster meat. Though tropomyosin is found in the muscles of other animals as well, the allergy causing protein fragment is unique to crustacea, the class of animals to which prawns, crabs and lobsters belong.

The scientists are now trying to identify the gene responsible for producing tropomyosin, so that they can start large-scale production of the allergen using genetic engineering techniques. This allergen can then be used as a vaccine to sensitise people against prawn and related allergies.