Sunlight exposure can precede the onset of recurrent eruptions of cold sores. UVB radiation appears to reduce the effectiveness of the immune system – in the case of cold sores it can no longer keep the virus Herpes simplex under control which results in re-activation of the infection. A study in the United States looked into the effect of sunscreen application on the incidence of cold sores. Of 38 patients, who recurrently suffer from Herpes simplex infections, 27 developed cold sores after exposure to UV radiation. In contrast, after the application of protective sunscreen, none of the patients developed cold sores. Therefore, beyond limiting skin-damaging effects, sunscreen may be effective in the prevention of sunlight-induced recurrent eruptions.
Research in recent years has provided increasing evidence that exposure to environmental UV levels can alter the activity and distribution of some of the cells responsible for triggering immune responses in humans. As a consequence sun exposure may enhance risk of infection or reduce the body's defences against skin cancer. Especially in countries of the developing world, high UV levels may decrease the effectiveness of vaccinations.
It has also been suggested that UV radiation may promote cancer in two distinct ways: by directly inducing DNA damage and by weakening the immune system. To date little research has been undertaken to describe the potential influence of immunomodulation on the development of cancer.