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Hypersensitivity reactions to contrast dye are quite common ranging from 4-12% with the use of ionic (hyperosmolar) contrast dye and 1-3% with nonionic contrast. Most reactions to hyperosmolar contrast dye are nonallergic or anaphylactoid (Type II) reactions. In fact, it is rare to have true anaphylactic (Type I) reactions to iodine mediated by antigen-IgE cross-linking with subsequent histamine release. Anaphylactoid reactions have a complex mechanism involving complement activation, direct degranulation of mast cells and basophils, and modulation of enzymes and proteolytic cascades but are not IgE-mediated. Clinically, anaphylactic and anaphylactoid reactions are indistinguishable causing urticaria, nausea, vomiting, and wheezing. Hypersensitivity reactions to contrast dye can often be prevented or lessened with the use of prophylactic corticosteroids and antihistamine medications. Type III or immune complex mediated reactions are seen in autoimmune syndromes including rheumatoid arthritis and systemic lupus erythematosus. Type IV or delayed-type hypersensitivity reactions involve the activation of T-cells and an example is the delayed hypersensitivity reaction to poison ivy.