C3 (complement factor)
| C3 (complement factor) | ||
|---|---|---|
| Identifier | ||
| Gene name (s) | C3 AHUS5, ARMD9, ASP, C3a, C3b, CPAMD1, HEL-S-62p | |
| External IDs | ||
The complement component C3 , also referred to as complement factor C3 , is a protein of the complement system that is part of the humoral immune response, ie it is used to lyse cells and bacteria by means of antibodies. C3 plays a central role in this process.
There are three different ways of complement activation that converge at complement factor C3: the classical way, the alternative way and the lectin way.
physiology
The factor C3 is inactive in the body and will only change into the active form after it has been split. During proteolysis, the C4b2b complex (C3 convertase) cleaves the protein in the traditional way into a smaller polypeptide C3a and a larger C3b.
Part C3b is involved in the cascade of the complement system and modifies the enzyme complex to form C5 convertase. It is bound away from the C4b2b complex and exposes a structure that can react with receptors on neutrophils and mast cells. As a result, the C4b-laden foreign particle can be phagocytosed.
C3a is an anaphylatoxin and promotes the degranulation and release of histamine from mast cells, leads to the contraction of smooth muscles and increased vascular permeability. This can lead to itching and urticaria.
Structure and genetics
The complement factor C3 is a glycoprotein consisting of an α and β chain of 185 kDa and is one of the plasma proteins. It is the complement factor with the highest serum concentration. The normal range is between 0.9 and 1.7 g / L in plasma.
The C3 coding C3 - gene is located in humans on chromosome 19 .
regulation
Several control proteins directly or indirectly regulate the formation of C3 convertase. The natural inhibitor C1INH inhibits the activity of C1 esterase. Proteins such as the C4 binding protein or the cell surface protein DAF (decay accelerating factor) can attach to C4 and thus prevent its binding to C2.
Clinical significance
Complement defects can lead to serious immune system disorders or life-threatening diseases. Pathological effects of complement activation can be found in anaphylactic reactions, thrombosis, transplant rejection and autoimmune diseases.
The serum concentrations of the complement factors have neither a high specificity nor a high sensitivity for certain diseases and are not determined routinely. Low concentrations of C3 are found in autoimmune diseases such as glomerulonephritis or systemic lupus erythematosus (SLE), with low serum concentrations reflecting increased disease activity.
Hereditary deficiencies of C3 have been described. This can lead to increased bacterial infections (e.g. with Neisseria ).
literature
- General microbiology. Editor: Georg Fuchs, founder: Hans G. Schlegel, Thieme Verlag, 8th edition, 2007
Individual evidence
- ↑ Remidiotti, MJ, Bianchetti, MG, Penzien JM et al: Glomerulonephritis with transient C3 hypocomplementemia and endomesengial glomerulonephritis in childhood. Switzerland Med Wschr 122: 1803-9 (1992)