The protective role of Langerhans' cells and sunlight in multiple sclerosis.

The low prevalence of multiple sclerosis in equatorial areas may be the consequence of a physiological immunosuppression induced by ultraviolet radiation on the epidermal Langerhans' cells, which are the antigen-presenting cells. Multiple sclerosis appears to be the consequence of an immune response to an environmental antigen during childhood in the absence of two protective factors: a genetic factor conferred by the HLA haplotypes of Langerhans' cells, and an environmental factor conferred by ultraviolet radiation in sunlight. In the presence of these two factors, T lymphocytes are physiologically immunosuppressed. Their association may play a key role in protection against multiple sclerosis. The varying combinations of these two factors may provide an explanation for the differences in incidence and prevalence of the disease noted within a single region or from one region to another throughout the globe. The hypothetical pathogenic mechanism developed in this article opens up new fields of research, particularly those concerning its therapeutic applications.