Expression of membrane HCD proteins and signaling. The Ig-α/Ig-β heterodimer is present on the cell surface and competent to mediate signals on aggregation, in the absence of any HC.⁹²  In the absence of LC (left), the HC is retained in the endoplasmic reticulum by the chaperone Ig binding protein (BiP).⁸⁵  V-less heavy chains can get to the surface (black arrow) in the absence of LC and spontaneously dimerize/aggregate in the absence of antigen (right), generating an antigen-like signal (green arrow),^66,79  whereas normal HC must pair with LC to be displayed on the cell surface^94,119  (but there are exceptions^39,40,98,120 ). Normal, unligated BCR generates a signal (yellow arrow),⁷¹  which, according to one model, might also involve some degree of spontaneous dimerization/aggregation.¹²¹