The Underlying Causes of Allergic Skin Reactions
Irritated skin can be caused by a variety of factors. These include immune system disorders, medications, and infections. When an allergen is responsible for triggering an immune system response that affects the skin, then it is considered an allergic skin condition, or skin allergy. There are several types of skin allergy.
Atopic Dermatitis, aka Eczema
Eczema is the most common skin condition, especially in children. It affects one in five infants, but only around one in fifty adults. It is now thought to be due to “leakiness” of the skin barrier, which causes it to dry out and become prone to irritation and inflammation by many environmental factors. Also, some people with eczema have a food sensitivity which can make eczema symptoms worse.
Hives are an inflammation of the skin that is triggered when the immune system releases histamine.
In about half of all people with severe atopic dermatitis, the disease is due to inheriting a faulty gene in their skin, the filaggrin gene. Unlike with urticaria (or hives), the itch of eczema may be caused by things other than histamine, so antihistamines may not control the symptoms. Eczema is often linked with asthma, allergic rhinitis (also called hay fever), and food allergy. This order of progression is called the atopic march.
Allergic Contact Dermatitis
Allergic contact dermatitis occurs when your skin comes in direct contact with an allergen. For instance, if you have a nickel allergy and your skin comes in contact with jewelry made with even a very small amount of nickel, you may develop red, bumpy, scaly, itchy, or swollen skin at the point of contact.
Coming in contact with poison ivy, poison oak, and poison sumac can also cause allergic contact dermatitis. The red, itchy rash is caused by an oily coating covering these plants. The allergic reaction can come from touching them directly, or by touching clothing, pets, or even gardening tools that have come in contact with the oil.
Urticaria, aka Hives
Hives are an inflammation of the skin that is triggered when the immune system releases histamine. This causes small blood vessels to leak, which leads to swelling in the skin. Swelling in deep layers of the skin is called angioedema.
There are two kinds of urticaria: acute and chronic. Acute urticaria occurs at times after eating a particular food or coming in contact with a particular trigger. It can also be triggered by nonallergic causes, such as heat or exercise, as well as medications, foods, insect bites, or infections. Chronic urticaria is rarely caused by specific triggers, so allergy tests are usually not helpful. Chronic urticaria can last for many months or years. Although they are often uncomfortable and sometimes painful, hives are not contagious.
Angioedema is swelling in the deep layers of the skin. It is often seen together with urticaria. Angioedema many times occurs in soft tissues, such as the eyelids, mouth, or genitals. Angioedema is called acute if the condition lasts only a short time, such as minutes to hours. Acute angioedema is commonly caused by an allergic reaction to medications or foods. Chronic recurrent angioedema is when the condition returns over a long period of time. It typically does not have an identifiable cause.
Hereditary angioedema is a rare, but serious genetic condition involving swelling in various body parts, including the hands, feet, face, intestinal wall, and airways. It does not respond to treatment with antihistamines or adrenaline, so it is important for people with signs of hereditary angioedema to see a specialist.
If you have itchy skin, your doctor or an allergist/immunologist can help you uncover the cause and recommend an effective treatment plan.
Why Do We Itch?
Itching can be caused by many different things, including allergies, insect bites, dry skin, and illness. While most itches are merely bothersome or uncomfortable, excessive scratching can damage your skin’s protective barrier and expose your body to germs and infection.
Itch and pain are closely linked in the brain. The reflex to pain is to withdraw. The reflex to itch is to scratch. This reflex is a protective response developed to help animals remove parasites from their skin. That’s why even a slight movement of hairs is enough to make you want to scratch.
Itching is often triggered by histamine, a chemical in the body associated with immune responses. It causes the itch and redness you see with insect bites, rashes, and skin dryness or damage. Histamine is released by the body during allergic reactions, such as those to pollen, food, latex, and medications.
Types of Itch
- Pruriceptive itch is due to an allergic reaction, inflammation, dryness, or other skin damage. It is seen in atopic dermatitis (or eczema), urticaria (or hives), psoriasis, drug reactions, mite bites, and dry skin. This type of itch is often treated with antihistamines and other drugs that alter the immune reaction.
- Neuropathic itch is caused by damage to the nervous system. It is usually accompanied by sensations of numbness and tingling. This type of itch is seen after shingles, after a stroke or burn injury, and in notalgia parasthetica (a sensory neuropathy that results in an area of itchy skin, usually on the back). It is treated with non-narcotic analgesics and capsaicin.
- Neurogenic itch is seen in chronic liver and kidney disease in response to opioid neuropeptides. It is treated with narcotic and non-narcotic analgesics.
- Psychogenic itch is induced in response to the chemicals serotonin and norepinephrine. These chemicals influence stress, depression, and delusional parasitosis (a disorder in which a person believes they are infested with parasites). Psychogenic itch is treated with antidepressants and antipsychotic medications.
Source: American Academy of Allergy, Asthma & Immunology, aaaai.org
This article was originally published in Coping® with Allergies & Asthma magazine, Fall/Winter 2017-2018.