Expert Lecture Details IgM and IgA Assembly, Complement Activation and FcRL5 Receptor Structure

An ImmuneZoom expert lecture on the Immune Zoom YouTube channel covered the structural biology of IgM and IgA assembly, secretion and complement activation. Key points include how the J chain and IgM tailpiece drive pentamer formation and interact with the polymeric Ig receptor for mucosal transport, how the stable binding of CD5L at the pentamer gap blocks that transport without affecting complement activation or FcμR binding, and how shortening the human IgM tailpiece to 11–16 residues or substituting the Xenopus IgX tailpiece yields efficient hexamers with stronger C1q recruitment. The lecture also described FcRL5 as an IgG Fc receptor that binds two adjacent IgG Fc regions at 1:2 stoichiometry and mediates lysosomal internalization, plus engineered hexameric IgG targeting CD20 showing enhanced complement-dependent cytotoxicity in vitro.