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A complement receptor is a receptor of the complement system, a part of the mediated innate immune system. Complement receptors are responsible for detecting pathogens by mechanisms not mediated by antibodies. Complement activity is not antigen sensitive, but can be triggered by specific antigens. Therefore complement (a group of proteins in the serum that help achieve phagocytosis and lysis of antigens) is also part of the humoral immune system.

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Complement receptors

Complement receptors are integral membrane proteins that bind fragments, which have been described in various cell types (Ross, 1985; Sim, 1986; Kinoshita, 1991; Sompayrac 2008). These receptors play a role in the regulation of cell growth and differantiation and other cellular functions. These receptors control abnormal activation of complement system. The complement system is composed of about twenty different proteins that work together to destroy invaders and signal other immune system players that attack is on.

The best studied are those that regulate C3 protein function, C3 receptors. In the human body these proteins are mainly produced by the liver, and are present in high concentrations in blood and tissues.

C3 molecules, the most aboundant complement proteins are continually being broken into a smaller level of degraded fragments of proteins. Distinct receptors exist for each of the degradation fragments of C3. Moreover, the C3 fragments are generally capable of interacting with more than one type of C3 receptor but with varying degrees of affinity.

C3b a very reactive protein fragment created by "spontaneous" cleavage. It can bind to either of two chemical groups (amino or hydroxyl). Since many of the proteins and carbohydrates that make up the surfaces of bacterial cells, viruses, etc. have amino acid or hydroxyl groups, there are many targets for these little C3b "grenades."

The integral membrane recptor that binds C3b, C4b and to a lesser degree iC3b, is referred to as complement receptor 1, or CR1. In humans the most common form of CR1 is 90kDa rodlike protein containing 30 repeating units termed short consensus repeats. The function of CR1 on B and T Lymphocytes has not been clarified.

A receptor for C3d fragment of C3dg exists on B lymphocates, B lymphoblastoid cell lines, and follicular dendriatic cells but is not present on granulocytes, monocytes, or macrophages (Fearon, 1983). This receptor, termed complement receptor (CR2), is reported to be important in B lymphocyte proliferation and differantiation. Also, it is on B lymphocytes to which the Epstein-Barr virus attaches during B cell infection, leading to infectious mononucleosis. CR2 interacts with each of the C3 fragments containnig residue 1209 to 1236 of C3d very strongly with C3d and, presumably, C3dg; strongly with iC3b; and weakly with C3b (Bylund, 1996).

A separate specific C3 receptor has been described to bind only to the C3 fragment iC3b. This receptor, referred to as complement receptor 3, or CR3, is present on granulocytes, mononuclear phagocytes, and natural killer (NK) cells. Ob phagocycotice cells, CR3 facilitates the phagocytic process in a parallel mode to CR1. Studies indicate that monocyte CR3 may play a synergistic role in enhancing the ingestion of particles coated with immunoglobin (IgG) and C3d (Gaither, 1987). This CR3 family of receptors is very important for cell adhesion and cell mediated cytotoxicity.

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