iC3b is a protein fragment that is part of the
complement system
The complement system, also known as complement cascade, is a part of the humoral, innate immune system and enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inf ...
, a component of the vertebrate
immune system
The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
. iC3b is produced when
complement factor I cleaves
C3b.
[Robbins Basic Pathology 8th ed 2007. R Cotran, S Robbins, V Kumar, J Perkins. W.B. Saunders Company.] Complement receptors on
white blood cells
White blood cells (scientific name leukocytes), also called immune cells or immunocytes, are cells of the immune system that are involved in protecting the body against both infectious disease and foreign entities. White blood cells are genera ...
are able to bind iC3b, so iC3b functions as an
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. ...
. Unlike intact C3b, iC3b cannot associate with
factor B, thus preventing amplification of the
complement cascade through the
alternative pathway. Complement factor I can further cleave iC3b into a protein fragment known as C3d.
References
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