IL-12 is a disulfide-linked heterodimeric protein (p70), composed of two subunits, p35 and p40, which are encoded by two different genes. Accumulating data indicate that p40 secretion precedes that of IL-12 expression. In addition, to its ability to covalently bind to p35 to form IL-12, p40 can bind to p19 to form IL-23, or can form a homodimer designated as IL-12 p80. Elevated levels of IL-12 p80 are correlated with macrophage recruitment and increased inflammation in asthma and respiratory viral infection models. Recombinant human IL-12 p80 is an 80kD disulfide linked homodimer consisting of two p40 chains of IL-12.