A protein, bound to the membrane of the endoplasmic reticulum (ER) of cells, that acts as a molecular chaperone to newly synthesized proteins of the immunoglobulin superclass. It associates with proteins, including T-cell receptors, MHC class I and II molecules, and immunoglobulins, that are in a partly folded or part-assembled state and retains them in the ER while they undergo further maturation.