Table 1.
Immunoglobulin receptors involved in transport and/or functions of effector cells or molecules at epithelial surfaces.
| Receptor | Ligand | Cell type | Characteristics |
|---|---|---|---|
| pIgR (102) | J chain in the context of IgA or IgM | Secretory epithelial cells | Transports IgA/IgM across epithelial cell layer |
| FcαRI (53, 103) | IgA1/2 (SIgA) | Myeloid cells | Major IgA receptor* |
| Fcα/μR (104) | IgM > IgA | B cells, macrophages | Type I transmembrane protein, mediates B cell endocytosis of antibody coated targets |
| FcμR/TOSO (26) | IgM | B- and T-cells | Highly specific for IgM. Currently undetermined function |
| β-galactosyltransferase (105) | pIgA, mIgA, SIgA | Liver, myeloid, intestinal epithelial cells | Unknown function, cation independent binding |
| Transferrin receptor (CD71) (106) | mIgA1 > pIgA | Renal mesangial cells | Unknown function, binding of IgA does not interfere with transferrin binding |
| SC-receptor (107, 108) | Secretory component | Eosinophils, basophils | Binding of IgA may cause degranulation |
| Asialoglycoprotein receptor (ASPGR) (109) | Terminal galactose and N-acetyl-d-galactosamine residues | Liver, myeloid, epithelial cells | Involved with IgA clearance from blood and liver |
| DC-SIGN (92, 94) | IgA/IgG | Sub-mucosal dendritic cells | Possible involvement in immune surveillance at mucosal surfaces, immune regulation |
| FcγRI | IgG | Monocytes, neutrophils, macrophages | These are the three major IgG receptor classes. A number of further isoforms have been described, of which all, except FcγRIIb, mediate cellular activation |
| FcγRII | IgG | Monocytes, neutrophils, eosinophils, basophils, B cells, platelets, macrophages, langerhans cells, endothelial cells of the placenta | Different functions are meditated on cross-linking by ligand or specific antibody, including phagocytosis, ADCC, cytokine release, superoxide production, and antigen presentation, except for the FcγRIIb which inhibits these ITAM-responses though its ITIM-signaling encoded in its cytoplasmic tail (110, 111) |
| FcγRIII | IgG | Monocytes, neutrophils, eosinophils, NK cells, T cells, macrophages, kidney mesangial cells, placental trophoblasts | |
| FcRL5 (and FcRL4) (112) | IgG (FcRL4 only IgG3 and IgG4) | B cells (FcRL4 on memory B cells) | ITIM-containing inhibitory receptors, that probably functions similar to FcγRIIb |
| TREM21 (98, 99) | IgG, IgM, others? | Mostly cells, high expression in myeloid cells | Cytoplasmic receptor, prompting ubiquitination-depended breakdown of IgG-opsonized particles |
| FcRn (64, 70, 72, 113) | IgG | Ubiquitous, high in epithelial cells, placental syncytiotrophoblasts, endothelial cells, monocytes, PMNs, dendritic cells | Transplacental transport, transepithelial transport, IgG regulation |
| FcδR (8) | IgD | Basophils, mast cells | Cytokine inducible. Other, less defined receptors have also been described on lymphocytes and basophils (32, 114). |
| TRIM21 (99, 101) | IgG, IgM, IgA | All cells, high on immune and endothelial cells | Intracellular Ig-receptor targeting cytoplasmic Ig-complexes for ubiquitin-dependent proteasome degradation |
| FcεRII (CD23) | IgE | B cells, enterocytes (90, 91) | In the gut CD23 promotes bidirectional transport of IgE and IgE complexes in the gut, thereby providing antigen sampling mechanism, suggesting a role for food allergies, and possibly protection against helminth infection |
*A number of even less established receptors for IgA on mesangial-, M-, epithelial-, and T-cells have also been reported.