IL12p40_C is the largely beta stranded C-terminal, D3, domain of interleukin-12p40 or interleukin-12B. This interleukin is produced on stimulation by macrophage-engulfed micro-organisms and other stimuli, when it dimerizes with interleukin-12p35 to form a heterodimer which then binds to receptors on natural killer cells to activate them to destroy the micro-organisms. This domain contains two disulfide bridges, one of which serves to bind p40 to p35 and the other to hold the beta strands within the domain together. The cupped shape of the p35 binding interface matches the elbow-like bend between D2 and D3 in p40. The domain is often associated with family fn3, pfam00041.