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 immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and at ...

, a component of the vertebrate

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...

. iC3b is produced when

complement factor I Complement factor I, also known as C3b/C4b inactivator, is a protein that in humans is encoded by the ''CFI'' gene. Complement factor I (factor I) is a protein of the complement system, first isolated in 1966 in guinea pig blood plasma, serum, t ...

cleaves

C3b C3b is the larger of two elements formed by the cleavage of complement component 3, and is considered an important part of the innate immune system. C3b is potent in opsonization: tagging pathogens, immune complexes (antigen-antibody), and apopto ...

.Robbins Basic Pathology 8th ed 2007. R Cotran, S Robbins, V Kumar, J Perkins. W.B. Saunders Company.

Complement receptors A complement receptor is a membrane-bound receptor belonging to the complement system, which is part of the innate immune system. Complement receptors bind effector protein fragments that are produced in response to antigen-antibody complexes or ...

white blood cells White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from mult ...

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 Complement factor B is a protein that in humans is encoded by the ''CFB'' gene. Function This gene encodes complement factor B, a component of the alternative pathway of complement activation. Factor B circulates in the blood as a single cha ...

, thus preventing amplification of the

complement cascade The complement system, also known as complement cascade, is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and ...

through the

alternative 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 and ...

. Complement factor I can further cleave iC3b into a protein fragment known as C3d.

References

* {{biochemistry-stub