Can consuming vitamin c help with itching and breathing difficulties acquired from shellfish allergy reaction?

Not enough: Shellfish allergy can be life-threatening, especially if you have breathing difficulties when you eat it. Taking vitamin c is not enough to guarantee that you will prevent a reaction. If you have a shellfish allergy, you should not eat it at all. It's also a good idea to see a doctor about this allergy and make sure you don't have others.

No: Not really. If you have the allergy symptoms that you are describing, then you need to be evaluated by a board certified allergist who will direct you to current recommendations and management of food allergy.