Heterodimerizes with IL12B to form the IL-12 cytokine or with EBI3/IL27B to form the IL-35 cytokine (PubMed:8943050, PubMed:8605935). IL-12 is primarily produced by professional antigen-presenting cells (APCs) such as B-cells and dendritic cells (DCs) as well as macrophages and granulocytes and regulates T-cell and natural killer-cell responses, induces the production of interferon-gamma (IFN-gamma), favors the differentiation of T-helper 1 (Th1) cells and is an important link between innate resistance and adaptive immunity (PubMed:1673147, PubMed:1674604, PubMed:8605935). Mechanistically, exerts its biological effects through a receptor composed of IL12R1 and IL12R2 subunits (PubMed:8943050). Binding to the receptor results in the rapid tyrosine phosphorylation of a number of cellular substrates including the JAK family kinases TYK2 and JAK2 (PubMed:7528775). In turn, recruited STAT4 gets phosphorylated and translocates to the nucleus where it regulates cytokine/growth factor responsive genes (PubMed:7638186). As part of IL-35, plays essential roles in maintaining the immune homeostasis of the liver microenvironment and functions also as an immune-suppressive cytokine (By similarity). Mediates biological events through unconventional receptors composed of IL12RB2 and gp130/IL6ST heterodimers or homodimers (PubMed:22306691). Signaling requires the transcription factors STAT1 and STAT4, which form a unique heterodimer that binds to distinct DNA sites (PubMed:22306691). Heterodimer with IL12B; disulfide-linked (PubMed:1674604, PubMed:10899108). This heterodimer is known as interleukin IL-12 (PubMed:1674604). Heterodimer with EBI3/IL27B; not disulfide-linked (PubMed:9342359). This heterodimer is known as interleukin IL-35 (PubMed:9342359). (Microbial infection) By pathogenic organisms, including Gram- positive and Gram-negative bacteria, parasites, viruses, and fungi.Down-regulated in response to enterovirus 71 (EV71) infection. Belongs to the IL-6 superfamily. Interleukin-12 entry