Fcalpha/micro receptor (Fcalpha/microR), CD351, is a type I transmembrane protein with an Ig-like domain in extracellular portion. It is a novel Fc receptor for IgM and IgA. Fcalpha/micro receptor mRNA has been found in thymus, spleen, liver, kidney, intestines, testes, placenta, and T cell leukemia cell lines. Fcalpha/micro receptor protein expression is detected on B cells, macrophages, and activated T cells. Fcalpha/micro receptor mediates primary B lymphocyte endocytosis of IgM-coated Staphylococcus aureus. It is involved in the primary stages of the immune response to microbes.