The immune system plays a key role in the development of an allergy. When the body perceives a certain substance as foreign and dangerous, it forms antibodies against it, known as immunoglobulins. This first stage is called sensitisation. Once the specific antibodies are present in the body, they react to the allergy-triggering substance (allergen) upon re-exposure, causing the allergy symptoms appear.

The chemical element iodine is a small molecule with a simple structure. It is so small that the immune system does not even notice it. An allergy to pure iodine is therefore impossible. Not only that, but iodine is essential for the body.

Iodine is not a foreign substance

“The human body needs iodine for the production of thyroid hormones. An iodine allergy would therefore be incompatible with life”, explains Ingrid Böhm of the University of Bern (Böhm et al. 2016). Thyroid hormones ensure that growth, bone formation, brain development and energy metabolism function normally.

Since humans cannot produce iodine, we need to get it from our food in sufficient quantities. Some foods, such as dairy products and seafood, naturally contain iodine. Iodine is added to many other food ingredients. This is done to ensure that the general population consumes a sufficient level of iodine.

Seafood allergy

“An allergy to fish, crustaceans or molluscs is not associated with their high iodine content at all”, explains Constance Katelaris from Western Sydney University in Australia (Katelaris et al. 2009). The allergy is triggered by specific proteins, such as parvalbumins in fish or tropomyosins in crustaceans and molluscs.

These proteins do not contain iodine. Nevertheless, the rumour persists that an allergy to seafood is caused by the iodine it contains. Katelaris, an allergy specialist, gives this example: At the University Children’s Hospital in Florida, in a survey among parents whose children had presented with a suspected seafood allergy, nine out of ten parents were convinced that iodine was the trigger of their child’s symptoms.

Medicinal products containing iodine

A number of medicinal products also contain iodine. These include disinfectants, X-ray contrast agents and medicines for fungal diseases. Some people have a sensitivity reaction to these substances, resulting in skin redness, swelling, itching, shortness of breath and even severe circulatory problems. These adverse effects are called pseudoallergic reactions because they do not involve any of the antibodies that are needed to trigger an allergic reaction.

Instead, the body is reacting to proteins in the substance that are not even associated with the iodine. If a medication triggers an adverse effect, it is referred to as hypersensitivity to the substance.

For example, before undergoing a computed tomography (CT) scan with a contrast agent, patients are often asked about their allergies – specifically, whether they have an ‘iodine allergy’, given that the contrast agents contain iodine. Nevertheless, it is still unclear which ingredients are the real triggers and whether they have anything at all to do with iodine.

At least 100 million CT scans are performed globally each year. Hypersensitivity reactions occur in one to three per cent of the procedures. “In order to protect patients from potential risks, we need to document the name of the contrast agent, along with a detailed description of the symptoms”, recommends Böhm of the University Hospital of Bern (Böhm et al. 2017).

Sources

1. Böhm I et al. “Iodine Allergy” – The Neverending Story. Rofo 2016;188(8):733-4.

2. Katelaris CH et al. ‘Iodine allergy’ label is misleading. Aust Prescr. 2009;32(5):125-8.

3. Böhm I et al. Iodinated Contrast Media and the Alleged “Iodine Allergy”: An Inexact Diagnosis Leading to Inferior Radiologic Management and Adverse Drug Reactions. Rofo. 2017;189(4):326-32.