|Themes > Science > Life Sciences > General Biology > Immunology > Recognition Systems in Immunity > Antigen Recognition: B Cells and Antibodies > Receptors for antibody|
There are various FcR with different properties. Monocytes and neutrophils express receptors (FcgammaR) for the Fc region of IgG. They constitutively express FcgammaRIIa and FcgammaRIIIa (monocytes) and FcgammaRIIIb (neutrophils). Mature tissue macrophages additionally express FcgammaRI, as do activated neutrophils. The principal difference between these receptors is that FcgammaRI is a high affinity receptor (Kd~10-9) and therefore can bind monovalent antibody/complexes while FcgammaRII and III are low affinity receptors (Kd~10-6) and thus only bind multivalent antibody-antigen complexes.
Opsonization refers to the coating of antigen with antibody that results in enhanced uptake of the antigen. There are two receptors capable of binding opsonized antigen. One is the FcgammaR, see above, the other is the receptor for one of the complement components C3b. If an antigen is coated with IgM antibody which has C3b bound to it then it will be opsonized via the C3b receptor. Ligation of the FcgammaR on phagocytes, during the process of phagocytosis of opsonized particles, leads to their activation and the enhanced uptake and degradation of antigen.
Antibody dependent cellular cytotoxicity (ADCC)
Neutrophils, eosinophils, phagocytes and NK cells all mediate ADCC. Ligation of the low affinity FcgammaRIII (CD16) molecule activates the lytic machinery. ADCC is only triggered by complexes of antigen and antibody. In this way, Fc regions form an array sufficient to increase the avidity of the interaction thus avoiding ADCC being triggered by free immunoglobulin.
Helminths cannot be killed by ADCC. Eosinophils, however, produce a basic protein which kills helminths and these cells express an Fc receptor for IgE. IgE provides some protection against helminths.
Immediate (type I) hypersensitivity
Caused by IgE antibodies. Ligation of the Fc epsilonR1 receptor on mast cells and basophils leads to the production of inflammatory and vasoactive mediators (histamine), lipid derived mediators (leukotrienes, prostaglandins, platelet activating factor etc.) and cytokines. This phenomenon manifests as allergy but is clearly of benefit in clearance of extracellular parasites.
Epithelial cells in the intestine and other secretory epitheilia (tear duct, salivary gland, lactating mammary gland etc) possess a receptor for polymeric Ig, which mediates secretion of antibody, predominantly IgA also IgM into the external secretions. Secreted IgA forms a very important defence against infection.
In Humans, maternal IgG can cross the
placenta. Babies have a high level of maternal IgG at birth. They also
acquire IgA (and some IgG) postnatally from breast milk, however these
remain in the gut lumen.
In all species maternal IgG,IgA are important both locally and systemically in protecting the neonate during the development of it's own immune system.
Co-ligation of the FcgammaRIIb receptor and surface Ig on B cells inhibits B cell activation and resulting antibody production thereby acting as a negative feedback loop.