Actinic prurigo is a chronic, itchy rash triggered by sunlight. People with actinic prurigo often have intensely itchy bumps or patches on sun‑exposed skin, and sometimes swollen lips or gritty, irritated eyes. It usually starts in childhood or adolescence and can flare each spring and summer. Mortality is not increased, but the itching and skin changes can affect daily life outdoors. Treatment often includes strict sun protection, topical corticosteroids, oral antihistamines, and sometimes medicines that calm the immune response.
Overview
Actinic prurigoSymptoms
Actinic prurigo causes intensely itchy, raised bumps on sun-exposed skin—often the face, neck, and arms. Flares follow sunlight and wind, and may swell, crack, or crust the lips and sometimes irritate the eyes. Scratching can leave marks or scars.
Outlook and prognosis
Most people with actinic prurigo improve when sun exposure is carefully managed and skin inflammation is treated early. Flares can come and go across seasons, especially in spring and summer. With consistent protection and dermatology follow-up, long-term control is common.
Causes and risk factors
Actinic prurigo often stems from an abnormal immune reaction to sunlight (UV). Risk rises with outdoor sun exposure, high altitude, childhood onset, family history or certain immune genes, darker or Indigenous American ancestry, and allergy/eczema tendency; sunscreen habits modify risk.
Genetic influences
Genetics play a meaningful role in actinic prurigo. Many people have specific HLA types, especially HLA-DR4, that raise risk and shape severity, while sunlight triggers symptoms. Family patterns occur, but not everyone with these variations develops actinic prurigo.
Diagnosis
Doctors make the diagnosis of Actinic prurigo by linking a sun‑triggered itchy rash to exam findings and history. They may order phototesting or a skin biopsy and rule out look‑alikes, such as lupus. Patch or blood tests are used selectively.
Treatment and drugs
Treatment for actinic prurigo aims to calm itchy, sunlight‑triggered rashes and protect skin. Plans often combine strict sun protection, moisturizers, antihistamines, steroid or calcineurin creams, and phototherapy; some need immunomodulating tablets or injections under specialist care. Regular follow‑up adjusts therapy by season and severity.
Symptoms
Sun-exposed skin can become intensely itchy, with small bumps and patches that show up soon after being outdoors. Symptoms vary from person to person and can change over time. Early symptoms of actinic prurigo often start in spring or summer and may be more noticeable on the face, lips, and arms.
A common sign is powerful itch that can feel impossible to ignore. It often starts soon after sun exposure and can disrupt focus or sleep. Repeated scratching can make the itch-rash cycle worse.
Small, firm bumps and patches appear on areas the sun hits, like the cheeks, nose, ears, and forearms. In actinic prurigo, flares can show up hours to a day after being outside. Cloud cover or window glass may not fully prevent a flare.
The lower lip may swell, crack, or crust after sun. Clinicians call this cheilitis, which means inflamed lips. It can make eating salty or spicy foods uncomfortable.
Redness, tearing, and a sandy or gritty feeling can develop, sometimes with light sensitivity. This eye discomfort may flare alongside skin symptoms of actinic prurigo. Wearing contact lenses can feel uncomfortable during a flare.
How people usually first notice Actinic prurigo?
People with actinic prurigo often first notice intense, itchy bumps or patches that appear or worsen after sun exposure, especially on the face, lips, neck, ears, and the backs of the hands. For many, this starts in childhood or adolescence and may come with swollen, cracked lips and small, scabbed areas from scratching, while people with darker skin tones may see darker or raised marks where the rash heals. Families often connect flare-ups to sunny days—even in winter at high altitude—making the “first signs of actinic prurigo” tied closely to light exposure rather than heat.
Types of Actinic prurigo
Actinic prurigo is a photosensitive skin condition, meaning symptoms flare after sun or sometimes indoor UV exposure. People with actinic prurigo can have different patterns of rash and itch, from mostly face and lip involvement to widespread bumps and nodules on sun-exposed skin. Symptoms don’t always look the same for everyone. Clinicians often describe them in these categories to help explain the different types of actinic prurigo people may experience.
Classic childhood-onset
Symptoms often begin in school-age years with intensely itchy bumps on the face, neck, and arms after sun exposure. Lips may crack or swell, and scratching can leave small scars. Flares tend to return every sunny season.
Adult-onset pattern
First episodes start later in life, sometimes after moving to a sunnier climate or spending more time outdoors. The rash and itch are similar but may be less tied to a childhood history. Some find symptoms reduce with careful sun protection.
Cheilitis‑predominant
The lips are the main site, becoming dry, cracked, and sore after UV exposure. People may also get small crusted patches around the mouth. Eating acidic or spicy foods can sting when lips are flared.
Nodular variant
Firm, very itchy nodules develop on chronically sun-exposed sites like forearms and the face. Scratching can thicken the skin and darken or lighten spots over time. This pattern may take longer to calm between flares.
Chronic conjunctival
The eyes are involved with redness, burning, or gritty sensation after light exposure. Lids and surrounding skin can also itch and swell. Sunglasses with side shields may help reduce flares.
Winter‑exacerbated
Symptoms flare even in cooler months when UV is lower but still present, especially at higher altitude. People may notice rashes after short outdoor periods or through window glass. Planning shade and protective clothing can make a difference.
High‑altitude/latitude
Flares are prominent in places with intense or reflected UV, such as mountains or snowy areas. The same person may have fewer symptoms at lower UV locations. Tracking where flares happen can clarify triggers and types of actinic prurigo.
Family‑linked pattern
Several relatives have similar sun‑triggered itching and bumps, suggesting inherited susceptibility. Onset can range from childhood to adulthood within a family. Not everyone in a family will have the same severity.
What genetics can tell you about symptoms
Certain HLA genes, especially HLA-DRB1*0407, raise the chance of intensely itchy, hive-like bumps and nodules after sun exposure, often on the face, arms, and lips. These variations may also link to swollen lips (cheilitis), conjunctivitis, and scarring from repeated scratching.
Causes and risk factors
Sunlight, especially ultraviolet light, is the main trigger for the skin reaction in actinic prurigo.
Genes set the stage, but environment and lifestyle often decide how the story unfolds.
A particular immune-system gene type, HLA-DRB1*0407, raises risk but does not guarantee disease, and it may run in families.
The condition is seen more often in people with Indigenous American ancestry and in some Latin American families, and it often starts in childhood.
Outdoor work or play, high altitude, and light reflected from water or snow are common triggers for actinic prurigo.
Environmental and biological risk factors
Actinic prurigo is an itchy, light-triggered rash that can flare with everyday sun exposure, even on cool or cloudy days. Below are environmental and biological risk factors for actinic prurigo that can raise the chance of flares or make them start earlier. Doctors often group risks into internal (biological) and external (environmental). These points focus on exposures and body factors linked to triggering or worsening the condition.
Ultraviolet A and B light can set off the immune itch-and-rash response in actinic prurigo. Flares often follow time outside, even on cool or hazy days. UVA can reach skin under light cloud cover.
UV levels rise with elevation, increasing exposure even during short outings. Mountain regions deliver stronger UV to exposed skin.
Living or traveling closer to the equator brings more intense year-round UV. This can make flares of actinic prurigo more frequent and longer lasting.
Sunlight bouncing off water, sand, snow, or concrete adds extra UV to skin. People may flare even under umbrellas or shade near these surfaces.
Genetic risk factors
Scientists have identified genetic risk factors for actinic prurigo, most notably certain immune-system HLA class II types such as HLA-DRB1*04:07. These HLA genes help the immune system recognize proteins, and certain versions seem to tilt the response toward the patterns seen in this condition. Risk is not destiny—it varies widely between individuals. Family clustering and higher rates in people with Indigenous American ancestry suggest a heritable, likely polygenic susceptibility.
HLA-DRB1*04: 07: This HLA class II allele shows the strongest known link to actinic prurigo. Carrying it increases susceptibility, especially in groups where this allele is common. Many carriers never develop the condition.
Other HLA alleles: Variants within the HLA-DR4 family and HLA-DQB1*03:02 have also been associated with actinic prurigo in some studies. Their effects appear smaller than DRB1*04:07 and may differ by ancestry. Together they suggest immune recognition pathways play a role.
Family history: Having a close relative with actinic prurigo raises your likelihood compared to the general population. Clusters across generations point to a heritable tendency rather than a single-gene cause. Exact risk for any one family can vary.
Ancestry patterns: Rates are higher among people with Native American or Indigenous Latin American ancestry. This likely reflects the higher frequency of HLA-DRB1*04:07 and related alleles in these populations. People within the same community can still have very different experiences.
Polygenic nature: Actinic prurigo appears polygenic, meaning several genes each add a small amount to risk. This can modify age at onset and severity among relatives. No single test can fully predict who will develop it.
Lifestyle risk factors
In actinic prurigo, everyday patterns of sun exposure and skin-care habits are key lifestyle risk factors for flares. Choices about when and how you spend time outdoors, what you wear, and what products you use can intensify or reduce symptoms. Understanding how lifestyle affects actinic prurigo helps you target changes with the biggest payoff. The elements below highlight practical lifestyle risk factors for actinic prurigo.
Being outside during peak UV hours (roughly 10 a.m.–4 p.m.) drives stronger immune reactions in sun‑exposed skin. Short, intense bursts of sun can trigger flares even on cool or cloudy days.
Jobs that keep you outside (farming, construction, lifeguarding) increase cumulative UV dose and flare frequency. Rotating tasks or building shade breaks can lower exposure without losing work time.
Indoor tanning delivers concentrated UVA that can provoke lesions and prolong active disease. Even brief sessions can set off delayed flares.
Skipping or under‑applying broad‑spectrum sunscreen allows enough UV to activate symptoms. Reapplication and adequate amounts are as important as SPF number.
Risk prevention
Actinic prurigo can’t be fully prevented, but you can lower the chance and severity of flares by limiting ultraviolet (UV) exposure and protecting skin and eyes every day. Even if you can’t remove all risks, prevention can reduce their impact. For many, building simple routines—like planning time outdoors and using protective gear—makes the biggest difference. Acting early when you notice the first itch or bumps after sun can also stop a small flare from becoming a long one.
Limit outdoor time when UV is strongest, usually 10 a.m.–4 p.m. Plan errands, exercise, and play for early morning or late afternoon.
Wear long sleeves, long trousers, and gloves if driving or cycling. Look for UPF-rated fabrics (ideally UPF 50+) to block UVA and UVB.
Use broad‑spectrum SPF 50+ on all exposed skin, including ears, neck, hands, and scalp edges. Apply about 1 ounce (30 mL) for full‑body coverage and reapply every 2 hours and after swimming or sweating.
Choose a wide‑brimmed hat (7–10 cm/3–4 in brim) to shade the face and neck. Use wraparound UV400 sunglasses to protect the eyes and eyelids.
Transmission
Actinic prurigo is not contagious—you can’t catch it from someone and you can’t give it to others through touch, coughing, sharing towels, or close contact. Instead, rashes are triggered by sunlight in people who are susceptible, and the tendency to develop actinic prurigo can run in families. Research points to certain immune-system genes that raise risk, especially in some Indigenous peoples of the Americas, but the genetic transmission of actinic prurigo is not straightforward. Having a parent or sibling with actinic prurigo does not mean you will get it, and many people with the condition have no family history. In short, there is no person-to-person transfer; how actinic prurigo is inherited is a risk tendency, not a direct inheritance of the disease itself.
When to test your genes
Test genetic risk when symptoms start young, cluster in families, or persist year‑round with sun‑triggered, intensely itchy bumps despite routine care. Consider testing if you’re from populations where actinic prurigo is more common (e.g., Indigenous peoples of the Americas) or planning pregnancy and want tailored prevention. Results can guide photoprotection, immunomodulator choices, and follow‑up.
Diagnosis
People with actinic prurigo often notice intensely itchy bumps and cracked or inflamed lips after time outdoors, sometimes even with brief sun exposure. Doctors usually begin by asking about when the rash shows up and where it appears, then examine the skin closely. The diagnosis of actinic prurigo combines your story, what’s seen on exam, and targeted tests to confirm photosensitivity and rule out look-alike conditions. It often starts in childhood and can be more common in certain families or backgrounds, so patterns can vary.
Your clinician asks when the itch and rash begin after sun, how long they last, and whether certain seasons are worse. They also note any lip inflammation or eye irritation that flares with light exposure. This timeline helps separate actinic prurigo from other itchy rashes.
The exam looks for small, very itchy bumps and scratch marks on sun-exposed areas like the face, neck, arms, and hands. Cheilitis (inflamed, chapped lips) and conjunctival redness may add clues. The distribution and chronicity are key features.
You’ll discuss daily routines, time outdoors, and how well sunscreen, hats, or shade help. Noting flares after indirect light or through windows can be informative. Travel to sunnier climates or high altitude may also be relevant.
Dermatology teams may expose small skin areas to controlled UVA and UVB light to check sensitivity. Reproducing the reaction supports a light-triggered condition. Results can help distinguish actinic prurigo from other photosensitive disorders.
Stages of Actinic prurigo
Actinic prurigo does not have defined progression stages. It tends to flare with sunlight and ease when exposure decreases, often worsening in spring and summer, so it doesn’t follow a steady, stepwise decline. Different tests may be suggested to help confirm the pattern and rule out look‑alike skin conditions. Diagnosis relies on your history and a skin exam of sun‑exposed areas; sometimes light testing, a small skin sample (biopsy), or blood tests are used, and sharing any early symptoms of actinic prurigo—like intense itch and small bumps after time outdoors—can guide the assessment.
Outlook and prognosis
Daily routines often adapt when actinic prurigo flares with sun or even bright indoor light—plans shift to earlier walks, wide‑brim hats, and long sleeves. Many people ask, “What does this mean for my future?”, and the short answer is that actinic prurigo tends to be long‑lasting but manageable. Symptoms often ebb and flow with seasons and latitude; winters or lower UV days can be quieter, while spring and summer bring more itch, bumps, and lip irritation. Doctors call this the prognosis—a medical word for likely outcomes.
With careful sun protection and the right treatment plan, many living with actinic prurigo keep skin symptoms controlled and continue work, school, and outdoor time with adjustments. Some people experience frequent flares and scarring, while others notice only occasional patches that calm with topical medicine and strict UV avoidance. Looking at the long-term picture can be helpful. Severe eye or lip involvement can leave lasting changes if untreated, but vision‑threatening complications are uncommon with regular care.
Actinic prurigo does not shorten life expectancy, and deaths from the condition are not reported; the main risks relate to quality of life, sleep, and skin changes from chronic scratching. Early symptoms of actinic prurigo that respond quickly—such as newly itchy bumps after sun—often predict better control when treatment starts promptly. In medical terms, the long-term outlook is often shaped by both genetics and lifestyle. Keep regular appointments—small adjustments can improve long-term health.
Long-term effects
Actinic prurigo is a long-lasting, sun‑sensitive skin condition that tends to flare after ultraviolet (UV) exposure and quiet down in between. Long-term effects vary widely, and everyone’s path looks different, and ongoing support can help. Early symptoms of actinic prurigo often begin in childhood with itchy bumps in spring or summer, and for some, the pattern continues into adult life. It usually isn’t dangerous, but ongoing itch, visible marks, and eye or lip involvement can affect comfort, sleep, and confidence.
Skin stays unusually sensitive to sunlight and sometimes even bright indoor light. Flares can happen quickly after UV exposure and last days to weeks. Over time, many learn their personal light thresholds.
Itchy bumps and patches return with sun exposure seasonally or year-round in sunny climates. The itch can be intense and hard to ignore. For some, actinic prurigo continues with repeated cycles over many years.
Repeated scratching can make skin thicker, rough, and more raised over time. These areas may catch on clothing and feel tender. The texture changes can take months to settle even after a flare ends.
Darker or lighter marks can linger where rashes were, especially after scratching. Some areas may scar, leaving small indented or firm spots. These color changes can be more noticeable on darker skin tones.
How is it to live with Actinic prurigo?
Living with actinic prurigo often means planning your day around sunlight, not just bright summer rays but even overcast light that can trigger intensely itchy, bumpy rashes on sun‑exposed skin and sometimes the lips and eyes. Many find they rely on wide‑brim hats, UV‑blocking clothing, wraparound sunglasses, and high‑SPF sunscreen, and may choose shade over spontaneous outdoor time, which can make social plans, school activities, or work outside more complicated. Flares can disturb sleep and concentration, and children with the condition may scratch to the point of skin breaks, so families often become partners in prevention—reminding about protective gear, helping apply treatments, and balancing safety with normal play. With consistent sun protection and guidance from a dermatologist, most people can reduce flares and keep routines, though it takes patience and support from those around them.
Treatment and drugs
Actinic prurigo is treated by protecting skin from sunlight and calming the immune-driven itch and rash it triggers. Daily sun protection is the foundation: broad‑spectrum sunscreen (SPF 50+), UV‑protective clothing, wide‑brim hats, wraparound sunglasses, and planning outdoor time to avoid peak sun. Doctors may add steroid creams or ointments for flares, nonsteroidal anti‑inflammatory creams for sensitive areas, and antihistamines or short courses of oral steroids to ease severe itch and swelling. For frequent or stubborn cases, options can include phototherapy (carefully dosed medical light), medicines that dial down the immune response such as tacrolimus ointment, methotrexate, thalidomide, or newer targeted therapies, with regular monitoring for side effects. Treatment plans often combine several approaches, and your doctor can help weigh the pros and cons of each option.
Non-drug treatment
Living with actinic prurigo often means planning your day around the sun to keep itching and flare-ups in check. Non-drug treatments often lay the foundation for staying active while protecting your skin. Recognizing early symptoms of actinic prurigo can help you start sun protection sooner. These approaches work best when they’re tailored to your routines, climate, and skin sensitivity.
Plan outdoor time in early morning or late afternoon when UV is lower. Seek shade and avoid reflective surfaces like water or snow.
Wear UPF 50+ long sleeves, trousers, and a wide-brim hat. UV-blocking sunglasses and neck protection help cover common flare areas.
Apply broad‑spectrum SPF 50+ to all exposed skin, including ears and the back of hands. Reapply every 2 hours and after sweating or swimming.
Add UV‑protective film to car and home windows to cut UVA exposure. Remember that regular glass blocks UVB but not much UVA.
Your genes may have an impact on how drugs work for you
Genes can affect how your immune system reacts to triggers like sunlight, which may change how well treatments for actinic prurigo work. They can also influence how your body processes medicines, shaping dose needs and the risk of side effects.
Pharmacological treatments
Treatment for actinic prurigo focuses on calming the itch, easing inflammation, and preventing flare-ups after sun exposure. Not everyone responds to the same medication in the same way. Your care team may combine creams with tablets and adjust dosing based on how your skin settles. Safety monitoring matters for some options, especially stronger prescription drugs.
Clobetasol or mometasone creams reduce redness and swelling during flares. They’re used in short bursts on the most active spots to limit skin thinning.
Tacrolimus or pimecrolimus creams help calm immune-driven inflammation without steroid side effects. They’re useful for delicate areas like the face and lips.
Cetirizine, fexofenadine, or hydroxyzine can ease itching, especially in the evening. For early symptoms of actinic prurigo like intense itch, these can take the edge off.
A brief course of prednisone can quiet a severe flare. Doctors taper the dose to reduce rebound and limit side effects.
Genetic influences
You may notice this sun-triggered, very itchy rash shows up in several relatives, or that early symptoms of actinic prurigo begin in childhood within the same family. Having a genetic risk is not the same as having the disease itself. Genes seem to set the threshold for how the immune system reacts to sunlight, and this helps explain why actinic prurigo can cluster in families and is more common in some Indigenous and Latin American populations.
Still, sun exposure is the spark, and many people with a similar family background never develop symptoms. Rather than a simple “yes or no” inheritance pattern, the tendency seems to come from several immune-related genes acting together—more like a dimmer than an on/off switch—plus factors like age and total sunlight exposure. Sharing your family history of actinic prurigo with your doctor can help tailor prevention and monitoring, especially for children who may be at higher risk.
Is Actinic prurigo genetic?
Genetics play a meaningful role in actinic prurigo. Many people have specific HLA types, especially HLA-DR4, that raise risk and shape severity, while sunlight triggers symptoms. Family patterns occur, but not everyone with these variations develops actinic prurigo.
Pharmacogenetics—how genetics influence drug effects
Genetics mainly help explain who is more likely to develop Actinic prurigo, but they rarely change the first steps of care like sun protection, soothing creams, or antihistamines. Certain immune-system gene patterns, found more often in some families and populations, raise the chance of Actinic prurigo; even so, treatment choices are guided by your symptoms, skin findings, and how often flares occur. Genetic testing can sometimes identify how your body processes certain medicines and guide safer dosing. Before starting azathioprine, for example, many clinicians check TPMT or NUDT15 activity to reduce the risk of low white blood cells; for cyclosporine, genes can affect drug levels, but doctors usually adjust the dose through regular blood monitoring. There isn’t a standard genetic test to predict side effects from thalidomide, a medicine used only in difficult cases, so strict safety steps—pregnancy prevention, nerve checks, and close follow-up—remain essential. In short, genes may fine‑tune drug selection and dose for some people with Actinic prurigo, but day‑to‑day management still depends on avoiding sun triggers and adjusting medicines to your individual response.
Interactions with other conditions
When another health issue is in the mix, flares can last longer and the itch may feel harder to control. People with actinic prurigo often also have allergic conditions such as eczema, hay fever, or asthma, and this tendency toward allergy can ramp up skin inflammation during sunny months. Conditions that also cause light‑triggered rashes—such as polymorphous light eruption or cutaneous lupus—may look similar on the skin, so teams sometimes check for overlap if symptoms are atypical or severe. The presence of another illness does not always mean actinic prurigo will be worse, but it can change how your care plan is put together. Open, itchy lesions raise the chance of bacterial skin infections, and treating these quickly can help settle a flare. If you live with actinic prurigo along with a disease that increases photosensitivity, clinicians usually tighten sun‑protection strategies and review other treatments to avoid medicines that make light sensitivity worse. Coordinated care between dermatology, allergy, and your primary team helps keep day‑to‑day symptoms and early symptoms of actinic prurigo flares more manageable.
How genes can cause diseases
Humans have more than 20,000 genes, each carrying out one or a few specific functions in the body. One gene instructs the body to digest lactose from milk, another tells the body how to build strong bones and another prevents the body's cells from beginning to multiply uncontrollably and developing into cancer. As all of these genes combined are the building instructions for our body, a defect in one of these genes can have severe health consequences.
Through decades of genetic research, we know the genetic code of any healthy/functional human gene. We have also identified that in certain positions on a gene, some individuals may have a different genetic letter from the one you have. We call these hotspots "Genetic Variations" or "Variants" in short. In many cases, studies have been able to show that having the genetic letter "G" in the position makes you healthy, but having the letter "A" in the same position disrupts the gene function and causes a disease. Genopedia allows you to view these variants in genes and summarizes all that we know from scientific research, which genetic letters (Genotype) have good or bad consequences on your health or on your traits.
History
Throughout history, people have described intense, seasonal itching and rashes after time in the sun, especially on the face, lips, and arms. Families in high-altitude regions told stories of children who each spring developed rough, itchy patches and swollen, cracked lips that eased in winter, only to return the next year. These patterns hinted that sunlight, not heat alone, was the trigger, and that some families were more prone than others.
First described in the medical literature as a distinctive sun-sensitive rash in certain Indigenous communities of the Americas, actinic prurigo was initially defined by who was seen in clinics rather than by its full biological range. Over time, descriptions became broader, recognizing that actinic prurigo can affect people of many backgrounds and can show up in childhood or later in life. Doctors noticed that, unlike a simple sunburn, the itch is severe, the bumps can become thick and scaly from scratching, and the lips may be involved. Early reports called it a “familial spring eruption,” though the term has largely been replaced as understanding improved.
In recent decades, awareness has grown that actinic prurigo is a chronic, immune-driven sensitivity to ultraviolet light. Careful clinic records and family histories showed that it often runs in families, supporting a genetic susceptibility. At the same time, researchers showed that reducing UV exposure helps, and that treatments which calm an overactive immune response can ease the rash and itch. This helped separate actinic prurigo from other sun-related conditions and explained why standard sunburn advice was not enough for many living with it.
Advances in genetics and immunology added more detail without changing the basics: sunlight exposure sets off an outsized skin and lip reaction in susceptible people. Some groups have higher rates, and living at altitude or in very sunny climates can bring symptoms on earlier in the year. Yet people in temperate regions can have actinic prurigo too, and cloudy days do not always prevent flares because ultraviolet light still gets through.
Looking back helps explain why actinic prurigo was once thought rare or confined to certain regions. Access to specialty care, seasonal patterns, and overlap with other rashes blurred the picture. Today, clinicians recognize the early symptoms of actinic prurigo—intense itch, recurring sun-exposed bumps, and lip involvement—and consider the diagnosis sooner. Despite evolving definitions, the core story has stayed steady: a lifelong tendency to react strongly to sunlight, shaped by genes, environment, and the immune system, and now managed more effectively thanks to decades of careful observation.