Explanation of the causes and mechanisms of hives

<Things you should know> Correct knowledge about "hives" (1) Explanation of the causes and mechanisms of hives

About hives
Main causes of hives
The mechanism of hives
Common concerns about hives

Hives, which appear suddenly one day accompanied by intense itching, are a common ailment affecting many people, from children to adults. Because they gradually subside over time, some may self-diagnose and decide it's not worth going to the doctor. However, a lack of understanding of the mechanism behind hives can lead to worsening or chronic conditions. This article will provide a clear explanation of the causes and mechanisms of hives.

About hives

Hives, a condition that afflicts people of all ages and causes intense itching, are the subject of this article. First, let's explain the origin of the word "hives" and how it differs from eczema.

Glossary

The origin of the Japanese word for "urticaria" (蕁麻辛) comes from the name of the nettle plant, "jinma," which is native to Japan. This plant has small thorns on its stems and leaves, and at the base of each thorn is a sac containing histamine and formic acid. When the thorns are touched, this sac ruptures and gets into the skin, causing pain and itching. It is said that this is how the rash caused by nettles came to be called urticaria.

The difference between hives and eczema

Urticaria (hives) and eczema are often confused because both cause redness and itchiness, but they are similar yet distinct conditions. Eczema typically presents as small, red blisters on the skin's surface in specific areas, which tend to worsen over time. Urticaria, on the other hand, is characterized by the sudden appearance of raised, red wheals (wails) that usually subside within 24 hours. Unlike eczema, which is localized, urticaria can affect the entire body. The medications used to treat urticaria and eczema are also different, so it is crucial to distinguish between them correctly.

Main causes of hives

The causes of hives can be broadly divided into two categories: "allergic" and "non-allergic."

Allergic urticaria

It is caused by allergens (substances that cause allergies) such as food, dust mites, dust, and pollen.
Non-allergic urticaria

It is caused by familiar things such as heat, cold, and the surrounding environment.

Generally, allergic cases caused by oral ingestion are considered the most common, but in many cases, identifying the cause is extremely difficult, and the cause of approximately 70% of hives, whether allergic or non-allergic, remains unknown. In cases where the cause has been identified, it may be something familiar in daily life, an unusual pattern, or a combination of several factors.

For example, if someone develops hives every time they eat shrimp, then shrimp is the cause, and a blood test can identify it as hives caused by a shrimp allergy. However, in cases where the cause is unknown, treatment can be difficult.

The mechanism of hives

Let's explain the mechanism by which hives occur. Hives involve blood, blood vessels, skin, and histamine. Histamine is a substance called an "active amine" made up of carbon, hydrogen, and nitrogen. This histamine is also present in the brain and is responsible for maintaining wakefulness and neurotransmission related to memory, and it also has the effect of suppressing appetite. Therefore, it is known that taking antihistamines can cause drowsiness, difficulty concentrating, and increased appetite.

Blood flows through the blood vessels that circulate throughout the body. This blood is composed of red blood cells that carry enzymes, white blood cells involved in immunity and allergies, and a liquid called plasma. When hives develop, attention is drawn to mast cells, a type of white blood cell. The name "mast cell" is unrelated to obesity in the sense of being overweight; it refers to the fact that these cells hold onto allergy-causing substances such as histamine and leukotrienes.

These mast cells, when stimulated by changes in body temperature or scratching, release substances they contain into the bloodstream, causing itching and swelling. A portion of the blood vessel in that area swells up like a balloon. As a result of this swelling, plasma leaks out through gaps in the vessel wall, pushing up the skin and causing it to bulge. Additionally, the increased blood flow due to the expansion causes the skin to turn red.

Common concerns about hives

Since the cause is unknown in most cases, many people are understandably anxious about hives. Below, we will answer some frequently asked questions.

Which department should I go to at the hospital?

While hives are generally treated by a dermatologist, they can also be a symptom of other conditions such as collagen diseases, so adults should see an internal medicine specialist, and children should see a pediatrician.
Is hives contagious?

Since hives are not caused by infection, they themselves have no infectious source and are not contagious. However, there is also infectious urticaria, which is caused by infection with viruses or bacteria.
Are the bumps that appear after taking a bath hives?

It could be hives, but it's not always the case. The difference depends on the shape, the intensity of the itching, the area, and the progression of the condition. Even with hives, the condition can worsen after a bath.
Can hives appear on the face?

It can appear all over the body. It can appear on the face, and can appear anywhere on the body. In the case of photourticaria, it may appear on the face, which is easily exposed to light.
Can a predisposition to hives be cured?

It depends on the cause of the hives. Regarding the allergic constitution, it varies; some cases improve, while others remain unchanged.
If food is the cause, eggs may improve the condition, but shellfish and buckwheat may be more difficult to treat. Idiopathic urticaria has an unknown cause, so it may recur repeatedly. There is also hereditary urticaria, so whether or not it will heal depends on the cause.
How long does it take for hives to subside?

Symptoms usually subside within a few minutes to a few hours, and in most cases, they clear up within 24 hours. Conversely, if symptoms persist for more than 24 hours, it may not be hives. However, hives may appear in one area and then in another, appearing as if they have moved within 24 hours, but the symptoms in that area will disappear within 24 hours.
Is there a connection between hives and the liver?

There may be two meanings. Hives caused by liver disease and hives caused by alcohol or other substances that affect liver metabolism. In hives caused by collagen diseases or infections, the enzymes AST and ALT contained in the liver are elevated. It is difficult to prove a causal relationship between liver strain and hives, but some cases of hives occur in conjunction with hepatitis, an infectious disease.
How should I deal with the itching caused by hives?

If you're experiencing severe itching, you can try cooling the affected area to alleviate the itching. However, this is contraindicated in cases of cold urticaria. Therefore, you should also refrain from using topical medications while they are cold. The same applies if there is swelling. In these cases, oral antihistamines are generally prescribed to relieve the symptoms.
Furthermore, if you experience recurrent hives of unknown origin, treatment including continuous oral medication may be necessary.
Does this mean I can no longer eat foods that cause hives?

We often hear people say how difficult it is not being able to eat what they want. The basic rule for foods that cause hives is to avoid them. However, in some cases, oral immunotherapy, which involves continuous oral intake, may allow you to eat certain foods. For information on what to do if a food is the allergen, please refer to (2) Main Symptoms and Types of Hives.

Source: "The True Common Sense of Hives" by Kunihiro Kiyomasu (Medical Economics Co., Ltd.)

Supervised by: Dr. Kunihiro Kiyomasu, Head of Pediatrics, Yamato-Takada Municipal Hospital
He graduated from Kyoto University Faculty of Medicine in 1992 and from Kyoto University Graduate School in 1999. He holds a PhD in Medicine and is a certified specialist and instructor of the Japan Pediatric Society and the Japanese Society of Allergology. His publications include "Common Sense and Uncommon Sense in Atopic Dermatitis Treatment" (Iyakukeizai-sha) and "The 'True' Common Sense about Urticaria" (Iyakukeizai-sha).

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