This is the immunoglobulin-like domain (IGD) found in obesity receptors (ObR). ObR is a single membrane-spanning receptor belonging to the class I cytokine receptor family. All isoforms have an identical extracellular part consisting of six domains: an N-terminal domain (NTD), two CRH domains (CRH1 and CRH2), an immunoglobulin-like domain (IGD), and two additional membrane-proximal fibronectin type III (FN III) domains. ObR activation depends on the CRH2, IGD, and FN III domains, however the CRH2 domain is the major leptin-binding determinant in the receptor. The IGD and membrane-proximal domains have no detectable affinity for the ligand, but are nonetheless indispensable for receptor activation. Deletion of the IGD results in a receptor with wild-type affinity for leptin, but completely devoid of biological activity [1-2].

1: Structure of the human obesity receptor leptin-binding domain reveals the mechanism of leptin antagonism by a monoclonal antibody. Carpenter B, Hemsworth GR, Wu Z, Maamra M, Strasburger CJ, Ross RJ, Artymiuk PJ; Structure. 2012;20:487-497. PMID:22405007

2: The Leptin Receptor Complex: Heavier Than Expected?. Wauman J, Zabeau L, Tavernier J; Front Endocrinol (Lausanne). 2017;8:30. PMID:28270795