Mediates response to the active Hedgehog (Hh) protein signal in embryos, functioning upstream or at the level of patched (ptc). Homodimer. Heterotetramer; 2 iHog chains bind 2 hh chains when facilitated by heparin, heparin is required to promote high-affinity interactions between hh and iHog (By similarity). The first fibronectin type-III domain mediates a specific interaction with Hh protein, in vitro. The second fibronectin type-III domain is additionally required for in vivo signaling activity (By similarity). Belongs to the immunoglobulin superfamily. IHOG family.