Allergic reactions to fish. The codfish allergen is heat stable and resistant to proteolytic enzymes. In sensitive people, it elicits symptoms such as rhinitis, dyspnea, eczema, urticaria, nausea, and vomiting following ingestion of fish. Urticaria may occur after skin contact with fish. Shellfish can cause strong allergic reactions as well. Probably, fish families have a species-specific antigen as well as cross-reactive antigens. Antibodies and allergic reactions may be directed against a specific fish or multiple fish families.