They can be different-- and generally this seems to start the way any other "contact" reaction starts.
Basically, imagine OAS... only, er-- yeah, different body parts.
But then it's like OAS symptoms that don't get better, and spread slowly (well, slowly in my experience); like a wave on an incoming tide, it just sort of keeps... getting worse... and worse... and worse. Eventually, though it's just anaphylaxis.
Understand that I am referring to latex here, and that my experience with shellfish is probably too limited to be of much use in this context. DH has been quite careful.