Functions as a receptor for the Fc fragment of IgA and IgM. Binds IgA and IgM with high affinity and mediates their endocytosis. May function in the immune response to microbes mediated by IgA and IgM. Expressed in several tissues including thymus, spleen, liver, kidney, small and large intestine, testis and placenta. Expressed by oligodendrocytes, B-cells and macrophages but not granulocytes, T-cells or NK cells (at protein level). N-glycosylated.