Interleukin-12 (IL-12) is a 70–75 kDa heterodimeric cytokine composed of a 40 kDa heavy chain and a 35 kDa light chain. It is produced mainly by monocytes and macrophages as well as dendritic cells, primarily in response to stimulation with bacterial or viral antigens. IL-12 has also been described to be produced by non-hematopoietic cells such as keratinocytes and epidermoid carcinomas. IL-12 displays various effects on both NK and T cells, playing an essential role in the interaction between innate and adaptive immunity: It induces the production of IFN-γ alone or in synergy with IL-2, is a co-stimulator of both NK and T cell proliferation, and enhances cytolytic activity of both cell types. Moreover, IL-12 promotes the development of T helper 1 (Tʜ1)-specific immune responses by promoting the differentiation of CD4

cells towards a Tʜ1 phenotype while inhibiting the production of Tʜ2 cells. The antibody clone REA121 reacts with the p35 kDa chain and the p70 kDa heterodimer of human IL-12, but not with IL-12 p40. It recognizes the same epitope as clone B-T21.