Is it possible to extract the allergens from shellfish?
Allergic reaction to shellfish is typically caused by the protein tropomyosin. Since it is present in basically all of the muscle tissue of shellfish, extraction is not possible outside of a chemistry lab. Said lab extraction would destroy the food and likely render it inedible.

Update 

Just found this recent article which indicates that myosin and sarcoplasmic calcium-binding protein (SCP) are also responsible for some allergic reactions.