Actinic prurigo (AP) is a chronic, pruritic skin disease caused by an abnormal reaction to sunlight. In 1954, Escalona first described it in Mexico.  Lesions appear hours or days following sun exposure, contrary to what happens in solar urticaria, in which skin lesions appear minutes after UV exposure. It is commonly associated with cheilitis and conjunctivitis. [2, 3] See the images below.
No systemic or local photosensitizer is known in patients with actinic prurigo, and a hypersensitivity implicating immunoglobulin E (IgE) has not been demonstrated.
Actinic prurigo has many features of a type IV hypersensitivity reaction. Skin lesions associated with actinic prurigo are infiltrated with T lymphocytes, mostly CD4+, and some of the T-cells express activation markers.  Actinic prurigo falls in the category of autoimmune diseases because lymphocytes from patients have been proven to be stimulated in a thymidine incorporation assay when confronted with their own UV-irradiated keratinocytes or UV-irradiated epidermal homogenates. 
At this point, the antigen that provokes the inflammatory reaction is not clear, but an epidermal protein is believed to be transformed by UV exposure. In the series by Santos-Martinez et al,  the presence of transforming growth factor-beta interleukin 13, and interleukin 10 was demonstrated in a non–type-TH1, non–type-TH2 pattern, similar to what has been shown in lesions of psoriasis and in the synovial fluid of rheumatoid arthritis.
Another potentially important finding in the pathogenesis of actinic prurigo may be the fact that Langerhans cells in persons with actinic prurigo show resistance to UV exposure when compared with those in healthy individuals.  This same finding has been shown in patients with a similar disease, polymorphous light eruption (PLE). Because these cells are resistant to their demise after UV exposure, they might handle and deliver UV-modified cutaneous antigens to T cells in larger amounts or in a more persistent way; this process could cause or augment the inflammatory phenomenon that is observed in the skin of patients with actinic prurigo. The apoptotic mechanism in these cells may be somewhat altered, facilitating their survival.
On the other hand, the polyclonal cellular immune response found in biopsy samples from Mexican patients through Southern blot analysis may involve an imbalance linked to a specific hyperimmunity, in which the proportion of autoimmune cells is increased and the proportion of other cells is decreased. 
Although different series are searching for a specific HLA, studies have shown associations with B40 and CW3 alleles in some populations, especially Amerindians. [9, 10] For instance, in the Chimila Indians from Colombia,  a high frequency of HLA-Cw4 was found. However, in Cree Indians from Saskatchewan, Canada,  the most common antigens were HLA-A24 and HLA-Cw4.
Other studies have shown a strong association with HLA-DR4. The more precise finding appears to be in the Mexican series, in which HLA-DR4 DRB1*0407 is found in more than 90% of patients with actinic prurigo.  Another series also found HLA-DR4 DRB1*0407 in Colombian patients.  Related alleles such as DRB*0407 have been found in British populations,  and DRB1*14 has been found in the Inuit Indians of Canada. 
Additional to the role of HLA-DR4 DRB1*0407 in the genetic susceptibility to develop actinic prurigo, HLA-DRB1*1406 could also be involved, and HLA-DRB1*0802 may possibly have a protective role. The existence of an HLA-B39-DRB1*0407 haplotype suggests a susceptibility region within the sixth human chromosome, at least among Amerindians affected by actinic prurigo. 
English patients with PLE  have not shown an association with any HLA, which suggests that HLA-DR4 (DRB1*0407) could be used as a marker to distinguish PLE from actinic prurigo. Therefore, the association with HLA in actinic prurigo but not in PLE suggests that actinic prurigo represents an immunologically mediated disease with strong genetic determinants for its expression. [11, 13]
Actinic prurigo occurs in persons of all skin types, but its prevalence in the general population is unknown. It probably represents less than 5% of referrals to photodermatologic clinics.  Actinic prurigo is well known in the United States among Native Americans. [10, 11, 18, 19]
In Mexico, actinic prurigo represents 1.34% of consultations with pediatric dermatologists and 4% of consultations with general dermatologists.  Actinic prurigo is common in Mexico, Central America, and South America, and it is well known in Canada among Native Americans. [10, 11, 19] Actinic prurigo rarely occurs in Europe and Asia, where PLE (a disease with pathogenetic features similar to actinic prurigo) is more regularly seen. Isolated cases have been reported in France,  Germany,  Japan,  Singapore,  Thailand,  and Australia.  However, the prevalence rate of actinic prurigo in photodermatology clinics around the world varies from 0-5%. 
Actinic prurigo frequently affects admixture populations (eg, mestizos) of Latin America and American Indians with skin phototypes IV or V.
Actinic prurigo can occur at any age; however, one third of patients are children.  The impact on quality of life in this population is serious. By means of the children's dermatology life quality index (CDLQI), children with actinic prurigo show high scores and closer to those obtained in children with xeroderma pigmentosum. 
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