To whom Correspondence should be addressed:

histocompatibility complex; ssUV, solar simulated ultraviolet TNCB, tri-nitro-chloro-benzene; UV, ultraviolet; 2 Immunosuppressive doses of solar simulated ultraviolet (UV) radiation activate lymph node B cells that can suppress primary immunity by inhibiting the function of dendritic cells. The aim of this study was to determine the waveband responsible for activation of these suppressor B cells. We exposed C57BL/6 mice to various doses of either UVA or UVB radiation and analysed the number and activation state of lymph node antigen presenting cells. Immunosuppressive doses of UVB but not UVA activated B cells as assessed by MHC II expression and doubled their numbers in draining lymph nodes. Higher doses of UVA that were not immunosuppressive actually suppressed B cell activation. Our results show that UVA and UVB suppress systemic immunity via different mechanisms. Lymph node B cells are activated in response to immunosuppressive doses of UVB but not UVA. Thus the activation state of lymph node APC appears to be important for UV immunomodulation.