Classes of Fc receptor
There are several types of Fc receptors based on the antibodies they recognize. For example, Fc receptor that bind IgG is called Fc gamma receptor (FcγR), Fc receptor that bind IgE is called Fc epsilon receptor (FcεR) and Fc receptor that bind IgA is called Fc alpha receptor (FcαR).
Depending on the affinity of receptor, Fc receptor can be classified as high-affinity receptor and low-affinity receptor. High-affinity Fc Receptor can bind non-complexed monomeric immunoglobulins. Low-affinity Fc Receptor do not bind monomeric immunoglobulins. Thus, Fc gamma receptor can be sorted as high-affinity receptor FcγRI, low-afffinity receptor FcγRII and FcγRIII. Fc epsilon receptor can be sorted as high-affinity receptor FcεRI, low-affinity receptor FcεRII.
Neonatal Fc receptor is kind of MHC class-I receptor. Neonatal Fc receptor is also called FCGRT. The neonatal Fc receptor was first discovered in rodents. Further studies revealed a similar receptor in humans. In rodents, neonatal Fc receptor in the neonatal gut epithelium and fetal yolk sac transports maternal IgG to the neonate and in humans neonatal Fc receptor mediates maternal-fetal IgG transport across the placenta.
FCRL is Fc Receptor-Like protein and is a subfamily of five Ig superfamily members. FCRL is also called Fc Receptor Homolog (FCRH). FCRLs have sequence homology with the classical Fc receptor for IgG. FCRL1 is an intrinsic activation molecule that has the potential to augment BCR-induced activation. FCRL1 has significant activation potential and may serve as an activation coreceptor on B cells.
FCAMR is a Fc receptor that binds to IgM and IgA. FCAMR is also called Fcα/μR. FCAMR has high affinity with IgM and Middle affinity with IgA. FCAMR is expressed on B cells, Mesangial cells and Macrophages, mediating endocytosis and induction of microbe killing.