A complement receptor is a membrane-bound

receptor Receptor may refer to: * Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ...

belonging to the complement system, which is part of the innate immune system. Complement receptors bind

effector Effector may refer to: *Effector (biology), a molecule that binds to a protein and thereby alters the activity of that protein * ''Effector'' (album), a music album by the Experimental Techno group Download * ''EFFector'', a publication of the El ...

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

fragments that are produced in response to antigen-antibody complexes or damage-associated molecules. Complement receptor activation contributes to the regulation of

inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...

leukocyte extravasation Leukocyte extravasation (also commonly known as leukocyte adhesion cascade or diapedesis – the passage of cells through the intact vessel wall) is the movement of leukocytes out of the circulatory system and towards the site of tissue damage or ...

, and

phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

; it also contributes to the adaptive immune response. Different complement receptors can participate in either the

classical complement pathway The classical complement pathway is one of three pathways which activate the complement system, which is part of the immune system. The classical complement pathway is initiated by antigen-antibody complexes with the antibody isotypes IgG and ...

, the

alternative complement pathway The alternative pathway is a type of cascade reaction of the complement system and is a component of the innate immune system, a natural defense against infections. The alternative pathway is one of three complement pathways that opsonize an ...

, or both.

Expression and function

White blood cells, particularly

monocyte Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and conventional dendritic cells. As a part of the vertebrate innate immune system monocytes also ...

s and macrophages, express complement receptors on their surface. All four complement receptors can bind to fragments of

complement component 3 Complement component 3, often simply called C3, is a protein of the immune system. It plays a central role in the complement system and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called ''C3''. Function ...

complement component 4 Complement component 4 (C4), in humans, is a protein involved in the intricate complement system, originating from the human leukocyte antigen (HLA) system. It serves a number of critical functions in immunity, tolerance, and autoimmunity with ...

coated on pathogen surface, but the receptors trigger different downstream activities. Complement receptor (CR) 1, 3, and 4 function as

opsonin Opsonins are extracellular proteins that, when bound to substances or cells, induce phagocytes to phagocytose the substances or cells with the opsonins bound. Thus, opsonins act as tags to label things in the body that should be phagocytosed (i.e. ...

s which stimulate

, whereas CR2 is expressed only on B cells as a

co-receptor A co-receptor is a cell surface receptor that binds a signalling molecule in addition to a primary receptor in order to facilitate ligand recognition and initiate biological processes, such as entry of a pathogen into a host cell. Properties The ...

Red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...

s (RBCs) also express CR1, which enables RBCs to carry complement-bound

antigen-antibody complex An immune complex, sometimes called an antigen-antibody complex or antigen-bound antibody, is a molecule formed from the binding of multiple antigens to antibodies. The bound antigen and antibody act as a unitary object, effectively an antigen o ...

es to the

liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it ...

and

spleen The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .

for degradation. :a.B: B cell. E:

erythrocyte Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...

. Endo: endothelial cell. D: dendritic cell. FDC:

follicular dendritic cell Follicular dendritic cells (FDC) are cells of the immune system found in primary and secondary lymph follicles (lymph nodes) of the B cell areas of the lymphoid tissue. Unlike dendritic cells (DC), FDCs are not derived from the bone-marrow hema ...

. Mac: macrophage. MC:

mast cell A mast cell (also known as a mastocyte or a labrocyte) is a resident cell of connective tissue that contains many granules rich in histamine and heparin. Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a par ...

. M0:

. Pha:

phagocyte Phagocytes are cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek ', "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek ...

. PMN:

polymorphonuclear leukocyte Granulocytes are cells in the innate immune system characterized by the presence of specific granules in their cytoplasm. Such granules distinguish them from the various agranulocytes. All myeloblastic granulocytes are polymorphonuclear. They hav ...

Clinical significance

Deficits in complement receptor expression can cause disease. Mutations in complement receptors which alter receptor function can also increase risk of certain diseases.

References

External links

* {{Pattern recognition receptors Complement system Single-pass transmembrane proteins

Expression and function

Clinical significance

See also

References

External links