Shellfish allergy symptoms generally start within minutes to an hour after eating or having contact with shellfish. They may include: 
Hives
Itchy, irritated skin
Nasal stuffiness (congestion)
Swelling of the lips, face, tongue and throat, or other parts of the body
Wheezing or trouble breathing
Coughing and choking or a tight feeling in the throat
Belly (abdominal) pain, diarrhea, nausea or vomiting
Dizziness, lightheadedness or fainting
 Anaphylaxis Allergies can cause a severe, potentially life-threatening allergic reaction known as anaphylaxis. It can occur within seconds to minutes after exposure to something you're allergic to ⸺ and worsens quickly. An anaphylactic reaction to shellfish is a medical emergency. Anaphylaxis requires immediate treatment with an epinephrine (adrenaline) injection and a follow-up trip to the emergency room. If anaphylaxis isn't treated right away, it can be fatal. Anaphylaxis causes the immune system to release a flood of chemicals that can cause you to go into shock. Signs and symptoms of anaphylaxis include: 
A swollen throat or tongue or a tightness in the throat (airway constriction) that makes it difficult for you to breathe
Coughing, choking or wheezing with trouble breathing
Shock, with a severe drop in your blood pressure and a rapid or weak pulse
Severe skin rash, hives, itching or swelling
Nausea, vomiting or diarrhea
Dizziness, lightheadedness or fainting
