A protein with a core domain architecture consisting of an extracellular domain containing two copies of the Immunoglobulin domain (Pfam:PF00047), followed by a single-pass transmembrane region and a small intracellular domain. The active protein is a low affinity receptor for immunoglobulin gamma chain Fc region. Human II-a, II-b, and II-c represent a recent gene expansion and are equally related to mouse II, III, and IV. Human III-A and III-B are closely related and closer to mouse IV than to mouse III. [ PRO:WCB ]