protein interleukin-13 T-cell activation protein P600 IL-13 IL13 NC30 A protein that is a translation product of the human IL13 gene or a 1:1 ortholog thereof. Interleukin-13 is a pleiotropic cytokine which may be important in the regulation of the inflammatory and immune responses. It inhibits inflammatory cytokine production and synergises with IL-2 in regulating interferon-gamma synthesis. The sequences of IL-4 and IL-13 are distantly related.