What chemical in shellfish makes people allergic to eating shellfish?

Fish protein allergy: Most allergens are proteins, and the allergens in shellfish that set off allergic people are no exception. These are usually unique to the particular species of fish, but some cross-reactivity is possible. In the case of shrimp, the protein has been identified and it is called tropomyosin. http://www.nytimes.com/1993/12/28/health/studies-unmask-protein-culprit-behind-allergy-to-shrimp.html.

Tropomyosin: Tropomyosins are the family of chemicals that cause the vast majority of allergy to shellfish. Allergy to fish is mediated by chemicals known as parvalbumins. A few patients are actually allergic to both tropomyosins and parvalbumins. There are other chemicals, but these are the predominant ones.