The immune system can be altered by UV irradiation, leading to diminished immune response to infectious agents and some cancers.

Some cells of the immune system, called antigen-presenting cells, reside in the skin. Their function is to capture invading micro organisms and carry them to the lymph nodes, where the immune response is initiated. These cells can be damaged directly by UV radiation and will then no longer be able to initiate an immune response or produce an aberrant one.

Other cells in the skin produce chemical mediators that direct the immune response toward either immune suppression or active immunity. Exposing these cells directly to UV-B radiation stimulates the release of mediators that favour the development of immune suppression. Finally, a molecule in the keratin layer of the skin, urocanic acid, undergoes a chemical change in response to UV radiation, which allows it to trigger the release of chemical mediators from mast cells in the skin, which also divert the immune response toward a pathway of immune suppression.

What is the significance of UV-induced immune suppression for human diseases? The answer is not yet clear. Although there are some examples in which UV exposure increases susceptibility to and the severity of an infection, such as with herpes virus infections (cold sores, shingles), the full implications of the immunological effects are unknown. Numerous laboratory animal models of infectious diseases demonstrate that exposure to UV radiation at a critical time during infection can increase the severity and duration of the disease. Also, UV exposure during immunisation can reduce the effectiveness of vaccinations. How these observations may apply to human diseases remains a subject of intense interest and research.