Preferred Label : Complement Pathway;
Alternative definition : BIOCARTA: The complement pathway consists of a series of over thirty proteins in plasma
that are part of the immune response. Activation of the complement system lyses bacterial
cells, forms chemotactic peptides (C3a and C5a) attracting immune cells, and increases
phagocytotic clearance of infecting cells. Complement can also increase the permeability
of vascular walls and cause inflammation. Most complement proteins exist in plasma
as inactive precursors that cleave and activate each other in a proteolytic cascade
in response to three different mechanisms by which the complement system is activated,
the classical pathway, the alternative pathway and the lectin-induced pathway. These
three systems are distinct in the initiation of the proteolytic cascade but share
most of their components and all three converge in the creation of a C3 convertase
that cleaves the C3 complement protein, leading ultimately to the formation of the
membrane attack complex, MAC, a pore causing lysis of cells. The classical pathway
is activated by the recognition of foreign cells by antibodies bound to the surface
of the cells. The alternative and lectin-induced pathways are both antibody independent.
Proteolysis is triggered in the alternative pathway by the spontaneous activation
of C3 convertase from C3 and is triggered in the lectin-induced pathway by the recognition
of carbohydrates on the bacterial cell surface by mannan-binding protein, Mbp. In
addition to providing a key part of the response to bacterial infection, the complement
system can be involved in the response to fungi, viruses and protists. While activation
of the complement system is a key part of the immune system, it must also be kept
in check to prevent inappropriate or exaggerated responses. Twelve different proteins
have been identified that inhibit complement activation to control the system, including
Factor H, Factor I and C1 inhibitor. Deficiencies in components of the complement
system have been identified in humans that cause a variety of immune related disorders.
C3 deficiency is associated with recurrent bacterial infections, while a lack of C2
can cause antibody-antigen complexes to accumulate and cause the autoimmune disorder
systemic lupus erythematosus. People lacking C1 inhibitor have also been identified
and found to be prone to uncontrolled complement activation and dangerous swelling
through production of C3a and C5a anaphylatoxins. (This definition may be outdated
- see the DesignNote.);
NCIt note : The BIOCARTA Definition (ALT_DEFINITION) for this pathway concept was provided by
BioCarta. This property was not created by, nor is it maintained by the NCI Thesaurus
staff. Additionally, BioCarta is no longer updating its pathway data; thus, the BIOCARTA
Definition might be outdated or inaccurate. Please see the Terms and Conditions for
Use at http://www.biocarta.com/.;
Biocarta ID : h_compPathway;
Origin ID : C39033;
UMLS CUI : C1305430;
Semantic type(s)
- has_gene_product_element
- pathway_has_gene_element
