J chain dictates the recognition of IgA by FcRL4

Fc-receptor-like 4 (FcRL4) selectively recognizes systemic IgA. The cryo-electron microscopy structure of the FcRL4-Fc (IgA-Fc)-Joining (J) chain complex reveals that FcRL4 interacts with Fc-J in a 1:1 stoichiometry, with FcRL4 primarily interacting with the J chain. The binding of Fc-J to FcRL4 would hinder the binding of pIgR, but not FcRI. These findings provide new insights into IgA and emphasize significant differences between various IgA receptors.