A halo nevus is a mole distinguished by a surrounding white ring of depigmented skin. These are typically benign and appear most often in children and young adults, with an estimated prevalence of about 1% in the white-skinned population. They are commonly found on the torso and can appear as a single mole or in multiples. While the appearance of a white ring around a mole can be startling, it is usually a harmless phenomenon.
How the Immune System Creates a Halo Nevus
A halo nevus forms as a result of the body’s immune system targeting a mole. This process is considered an autoimmune response, where the body’s defense mechanisms mistakenly identify the pigment-producing cells, called melanocytes, within the mole as foreign. The reason for this targeted recognition is not fully understood, but it may be triggered by factors like sunburn or local injury to the mole.
This immune recognition leads to an attack by specific white blood cells, known as T-cells. These cells infiltrate the mole and the surrounding skin, releasing substances that destroy the melanocytes. The destruction of these pigment cells is what causes the skin around it to turn white, creating the characteristic halo. This reaction is highly specific to the melanocytes in that particular area.
The immune response is localized, affecting only the targeted mole and the immediate surrounding skin. This is why the depigmentation is confined to a sharp, circular border around the nevus. Researchers believe this focused immune activity is similar to other autoimmune conditions, which helps explain its connection to disorders like vitiligo.
The Lifecycle of a Halo Nevus
The development of a halo nevus follows a predictable, though often lengthy, series of stages. The process begins when a well-defined, symmetrical white halo appears around a pre-existing mole. This initial stage signals that the immune system has begun its work of breaking down the melanocytes. The halo itself is round or oval with a distinct border.
Over a period that can span from months to years, the central mole itself begins to change. It will gradually fade in color, often turning from dark brown to light brown, and then to pink. Concurrently, the mole may shrink in size as the nevus cells are slowly cleared away by the immune system. This regression is a sign that the process is advancing as expected for a benign halo nevus.
Eventually, the central mole may disappear completely, leaving behind only the white patch of skin. In many cases, this depigmented area will remain for some time. For some individuals, the final stage involves the repigmentation of the halo, where the skin gradually returns to its normal color, although this does not always happen.
Distinguishing a Halo Nevus from Skin Cancer
While a classic halo nevus is benign, the appearance of a halo can, in rare instances, be associated with melanoma. Distinguishing between a harmless nevus and potential skin cancer lies in the appearance of the central mole, not the halo itself. A benign halo nevus has a symmetric, evenly colored brown mole at its center, surrounded by a uniform ring of depigmentation.
A visit to a dermatologist is warranted if the central mole shows any atypical features. Warning signs that require a professional evaluation include:
- An irregular or asymmetrical shape
- Multiple colors such as black, dark brown, or red within the mole
- A border that is blurry or jagged
- Changes to the mole’s surface, such as itching, crusting, or bleeding
A dermatologist can use a special magnifying tool called a dermatoscope to examine the mole’s structure and color patterns more closely. If there is any suspicion, a skin biopsy may be performed to definitively rule out melanoma. While a halo phenomenon around a melanoma is uncommon, it is more concerning when it appears in adults, who have a slightly higher risk of the halo being associated with a melanoma elsewhere on the body.
The Connection Between Halo Nevi and Vitiligo
There is an association between halo nevi and vitiligo, a separate autoimmune skin condition characterized by patches of skin losing their pigment. The appearance of a halo nevus, particularly when multiple nevi are present, can precede or occur in individuals who already have vitiligo. This connection exists because both conditions share the same underlying autoimmune mechanism.
The presence of a halo nevus suggests a person may have a predisposition for this type of autoimmune activity. The primary difference between the conditions is the scale of the reaction. In a halo nevus, the attack is localized to a mole, while in vitiligo, it can affect skin anywhere on the body.
Having a halo nevus does not guarantee that a person will develop vitiligo. However, individuals with halo nevi, especially those with multiple lesions, have a higher incidence of vitiligo. Because of the shared autoimmune link, experts may recommend screening for related conditions, like thyroid dysfunction, in people who present with halo nevi.